diff --git a/conjugation/drill.html b/conjugation/drill.html
index 71b3478..e74b3e5 100644
--- a/conjugation/drill.html
+++ b/conjugation/drill.html
@@ -24,6 +24,7 @@
         <div>
           <h2>Conjugation types</h2>
           <ul>
+            <li><label><input type="checkbox" id="plain" checked> Plain</label></li>
             <li><label><input type="checkbox" id="polite" checked> Polite</label></li>
             <li><label><input type="checkbox" id="negative" checked> Negative</label></li>
             <li><label><input type="checkbox" id="past" checked> Past</label></li>
diff --git a/conjugation/drill.js b/conjugation/drill.js
index b1755d6..34dedf4 100644
--- a/conjugation/drill.js
+++ b/conjugation/drill.js
@@ -814,108 +814,108 @@ var words = {
 
 var transformations = [
 
-  { from: "negative", to: "plain", phrase: "affirmative", tags: ["negative"] },
-  { from: "past", to: "plain", phrase: "present", tags: ["past"] },
-  { from: "polite", to: "plain", phrase: "plain", tags: ["polite"] }, 
-  { from: "te-form", to: "plain", phrase: "plain", tags: ["te-form"] }, 
-  { from: "potential", to: "plain", phrase: "plain", tags: ["potential"] }, 
-  { from: "imperative", to: "plain", phrase: "plain", tags: ["imperative"] }, 
-  { from: "causative", to: "plain", phrase: "plain", tags: ["causative"] }, 
-  { from: "passive", to: "plain", phrase: "plain", tags: ["passive"] }, 
-  { from: "progressive" , to: "plain", phrase: "non-progressive", tags: ["progressive"] }, 
-  { from: "plain", to: "negative", phrase: "negative", tags: ["negative"] }, 
-  { from: "past negative", to: "negative", phrase: "present", tags: ["past", "negative"] }, 
-  { from: "polite negative", to: "negative", phrase: "plain", tags: ["polite", "negative"] }, 
-  { from: "te-form negative", to: "negative", phrase: "plain", tags: ["te-form", "negative"] }, 
-  { from: "potential negative", to: "negative", phrase: "plain", tags: ["potential", "negative"] }, 
-  { from: "imperative negative", to: "negative", phrase: "plain", tags: ["imperative", "negative"] }, 
-  { from: "causative negative", to: "negative", phrase: "plain", tags: ["causative", "negative"] }, 
-  { from: "passive negative", to: "negative", phrase: "plain", tags: ["passive", "negative"] }, 
-  { from: "progressive negative", to: "negative", phrase: "non-progressive", tags: ["progressive", "negative"] }, 
-  { from: "plain", to: "past", phrase: "past", tags: ["past"] }, 
-  { from: "past negative" , to: "past", phrase: "affirmative", tags: ["past", "negative"] }, 
-  { from: "polite past", to: "past", phrase: "plain", tags: ["polite", "past"] }, 
-  { from: "progressive past", to: "past", phrase: "non-progressive", tags: ["progressive", "past"] }, 
-  { from: "negative", to: "past negative", phrase: "past", tags: ["past", "negative"] }, 
-  { from: "past", to: "past negative", phrase: "negative", tags: ["past", "negative"] }, 
-  { from: "polite past negative", to: "past negative", phrase: "plain", tags: ["polite", "past", "negative"] }, 
-  { from: "progressive past negative", to: "past negative", phrase: "non-progressive", tags: ["progressive", "past", "negative"] }, 
-  { from: "plain", to: "polite", phrase: "polite", tags: ["polite"] }, 
+  { from: "negative", to: "plain", phrase: "affirmative", tags: ["plain", "negative"] },
+  { from: "past", to: "plain", phrase: "present", tags: ["plain", "past"] },
+  { from: "polite", to: "plain", phrase: "plain", tags: ["plain", "polite"] }, 
+  { from: "te-form", to: "plain", phrase: "plain", tags: ["plain", "te-form"] }, 
+  { from: "potential", to: "plain", phrase: "plain", tags: ["plain", "potential"] }, 
+  { from: "imperative", to: "plain", phrase: "plain", tags: ["plain", "imperative"] }, 
+  { from: "causative", to: "plain", phrase: "plain", tags: ["plain", "causative"] }, 
+  { from: "passive", to: "plain", phrase: "plain", tags: ["plain", "passive"] }, 
+  { from: "progressive" , to: "plain", phrase: "non-progressive", tags: ["plain", "progressive"] }, 
+  { from: "plain", to: "negative", phrase: "negative", tags: ["plain", "negative"] }, 
+  { from: "past negative", to: "negative", phrase: "present", tags: ["plain", "past", "negative"] }, 
+  { from: "polite negative", to: "negative", phrase: "plain", tags: ["plain", "polite", "negative"] }, 
+  { from: "te-form negative", to: "negative", phrase: "plain", tags: ["plain", "te-form", "negative"] }, 
+  { from: "potential negative", to: "negative", phrase: "plain", tags: ["plain", "potential", "negative"] }, 
+  { from: "imperative negative", to: "negative", phrase: "plain", tags: ["plain", "imperative", "negative"] }, 
+  { from: "causative negative", to: "negative", phrase: "plain", tags: ["plain", "causative", "negative"] }, 
+  { from: "passive negative", to: "negative", phrase: "plain", tags: ["plain", "passive", "negative"] }, 
+  { from: "progressive negative", to: "negative", phrase: "non-progressive", tags: ["plain", "progressive", "negative"] }, 
+  { from: "plain", to: "past", phrase: "past", tags: ["plain", "past"] }, 
+  { from: "past negative" , to: "past", phrase: "affirmative", tags: ["plain", "past", "negative"] }, 
+  { from: "polite past", to: "past", phrase: "plain", tags: ["plain", "polite", "past"] }, 
+  { from: "progressive past", to: "past", phrase: "non-progressive", tags: ["plain", "progressive", "past"] }, 
+  { from: "negative", to: "past negative", phrase: "past", tags: ["plain", "past", "negative"] }, 
+  { from: "past", to: "past negative", phrase: "negative", tags: ["plain", "past", "negative"] }, 
+  { from: "polite past negative", to: "past negative", phrase: "plain", tags: ["plain", "polite", "past", "negative"] }, 
+  { from: "progressive past negative", to: "past negative", phrase: "non-progressive", tags: ["plain", "progressive", "past", "negative"] }, 
+  { from: "plain", to: "polite", phrase: "polite", tags: ["plain", "polite"] }, 
   { from: "polite past", to: "polite", phrase: "present", tags: ["polite", "past"] }, 
   { from: "polite negative", to: "polite", phrase: "affirmative", tags: ["polite", "negative"] }, 
   { from: "polite progressive", to: "polite", phrase: "non-progressive", tags: ["polite", "progressive"] }, 
-  { from: "past", to: "polite past", phrase: "polite", tags: ["past", "polite"] }, 
+  { from: "past", to: "polite past", phrase: "polite", tags: ["plain", "past", "polite"] }, 
   { from: "polite", to: "polite past", phrase: "past", tags: ["past", "polite"] }, 
   { from: "polite past negative", to: "polite past", phrase: "affirmative", tags: ["polite", "past", "negative"] }, 
   { from: "polite progressive past", to: "polite past", phrase: "non-progressive", tags: ["polite", "progressive", "past"] }, 
-  { from: "negative", to: "polite negative", phrase: "polite", tags: ["polite", "negative"] }, 
+  { from: "negative", to: "polite negative", phrase: "polite", tags: ["plain", "polite", "negative"] }, 
   { from: "polite", to: "polite negative", phrase: "negative", tags: ["polite", "negative"] }, 
   { from: "polite past negative", to: "polite negative", phrase: "present", tags: ["polite", "past", "negative"] }, 
   { from: "polite progressive negative", to: "polite negative", phrase: "non-progressive", tags: ["polite", "progressive", "negative"] }, 
-  { from: "past negative", to: "polite past negative", phrase: "polite", tags: ["polite", "past", "negative"] }, 
+  { from: "past negative", to: "polite past negative", phrase: "polite", tags: ["plain", "polite", "past", "negative"] }, 
   { from: "polite past", to: "polite past negative", phrase: "negative", tags: ["polite", "past", "negative"] }, 
   { from: "polite negative", to: "polite past negative", phrase: "past", tags: ["polite", "past", "negative"] }, 
   { from: "polite progressive past negative", to: "polite past negative", phrase: "non-progressive", tags: ["polite", "progressive", "past", "negative"] }, 
-  { from: "plain" , to: "te-form", phrase: "て", tags: ["te-form"] }, 
-  { from: "te-form negative" , to: "te-form", phrase: "affirmative", tags: ["te-form", "negative"] }, 
-  { from: "te-form" , to: "te-form negative", phrase: "negative", tags: ["te-form", "negative"] }, 
-  { from: "negative" , to: "te-form negative", phrase: "て", tags: ["te-form", "negative"] }, 
-  { from: "plain" , to: "potential", phrase: "potential", tags: ["potential"] }, 
-  { from: "potential negative" , to: "potential", phrase: "affirmative", tags: ["potential", "negative"] }, 
-  { from: "potential" , to: "potential negative", phrase: "negative", tags: ["potential", "negative"] }, 
-  { from: "negative" , to: "potential negative", phrase: "potential", tags: ["potential", "negative"] }, 
-  { from: "plain" , to: "imperative", phrase: "imperative", tags: ["imperative"] }, 
-  { from: "imperative negative" , to: "imperative", phrase: "affirmative", tags: ["imperative", "negative"] }, 
-  { from: "imperative" , to: "imperative negative", phrase: "negative", tags: ["imperative", "negative"] }, 
-  { from: "negative" , to: "imperative negative", phrase: "imperative", tags: ["imperative", "negative"] }, 
-  { from: "plain" , to: "causative", phrase: "causative", tags: ["causative"] }, 
-  { from: "causative negative" , to: "causative", phrase: "affirmative", tags: ["causative", "negative"] }, 
-  { from: "causative passive" , to: "causative", phrase: "active", tags: ["causative", "passive"] }, 
-  { from: "causative" , to: "causative negative", phrase: "negative", tags: ["causative", "negative"] }, 
-  { from: "negative" , to: "causative negative", phrase: "causative", tags: ["causative", "negative"] }, 
-  { from: "causative passive negative" , to: "causative negative", phrase: "active", tags: ["causative", "passive", "negative"] }, 
-  { from: "plain" , to: "passive", phrase: "passive", tags: ["passive"] }, 
-  { from: "passive negative" , to: "passive", phrase: "affirmative", tags: ["passive", "negative"] }, 
-  { from: "causative passive" , to: "passive", phrase: "non-causative", tags: ["causative", "passive"] }, 
-  { from: "passive" , to: "passive negative", phrase: "negative", tags: ["passive", "negative"] }, 
-  { from: "negative" , to: "passive negative", phrase: "passive", tags: ["passive", "negative"] }, 
-  { from: "causative passive negative" , to: "passive negative", phrase: "non-causative", tags: ["causative", "passive", "negative"] }, 
-  { from: "causative" , to: "causative passive", phrase: "passive", tags: ["causative", "passive"] }, 
-  { from: "passive" , to: "causative passive", phrase: "causative", tags: ["causative", "passive"] }, 
-  { from: "causative passive negative" , to: "causative passive", phrase: "affirmative", tags: ["causative", "passive", "negative"] }, 
-  { from: "causative negative" , to: "causative passive negative", phrase: "passive", tags: ["causative", "passive", "negative"] }, 
-  { from: "passive negative" , to: "causative passive negative", phrase: "causative", tags: ["causative", "passive", "negative"] }, 
-  { from: "causative passive" , to: "causative passive negative", phrase: "negative", tags: ["causative", "passive", "negative"] }, 
-  { from: "plain", to: "progressive", phrase: "progressive", tags: ["progressive"] }, 
-  { from: "polite progressive", to: "progressive", phrase: "plain", tags: ["polite", "progressive"] }, 
-  { from: "progressive negative", to: "progressive", phrase: "affirmative", tags: ["progressive", "negative"] }, 
-  { from: "progressive past", to: "progressive", phrase: "present", tags: ["progressive", "past"] }, 
-  { from: "progressive", to: "progressive negative", phrase: "negative", tags: ["progressive", "negative"] }, 
-  { from: "negative", to: "progressive negative", phrase: "progressive", tags: ["negative", "progressive"] }, 
-  { from: "progressive past negative", to: "progressive negative", phrase: "present", tags: ["progressive", "past", "negative"] }, 
-  { from: "polite progressive negative", to: "progressive negative", phrase: "plain", tags: ["polite", "progressive", "negative"] }, 
-  { from: "progressive", to: "polite progressive", phrase: "polite", tags: ["progressive", "polite"] }, 
+  { from: "plain" , to: "te-form", phrase: "て", tags: ["plain", "te-form"] }, 
+  { from: "te-form negative" , to: "te-form", phrase: "affirmative", tags: ["plain", "te-form", "negative"] }, 
+  { from: "te-form" , to: "te-form negative", phrase: "negative", tags: ["plain", "te-form", "negative"] }, 
+  { from: "negative" , to: "te-form negative", phrase: "て", tags: ["plain", "te-form", "negative"] }, 
+  { from: "plain" , to: "potential", phrase: "potential", tags: ["plain", "potential"] }, 
+  { from: "potential negative" , to: "potential", phrase: "affirmative", tags: ["plain", "potential", "negative"] }, 
+  { from: "potential" , to: "potential negative", phrase: "negative", tags: ["plain", "potential", "negative"] }, 
+  { from: "negative" , to: "potential negative", phrase: "potential", tags: ["plain", "potential", "negative"] }, 
+  { from: "plain" , to: "imperative", phrase: "imperative", tags: ["plain", "imperative"] }, 
+  { from: "imperative negative" , to: "imperative", phrase: "affirmative", tags: ["plain", "imperative", "negative"] }, 
+  { from: "imperative" , to: "imperative negative", phrase: "negative", tags: ["plain", "imperative", "negative"] }, 
+  { from: "negative" , to: "imperative negative", phrase: "imperative", tags: ["plain", "imperative", "negative"] }, 
+  { from: "plain" , to: "causative", phrase: "causative", tags: ["plain", "causative"] }, 
+  { from: "causative negative" , to: "causative", phrase: "affirmative", tags: ["plain", "causative", "negative"] }, 
+  { from: "causative passive" , to: "causative", phrase: "active", tags: ["plain", "causative", "passive"] }, 
+  { from: "causative" , to: "causative negative", phrase: "negative", tags: ["plain", "causative", "negative"] }, 
+  { from: "negative" , to: "causative negative", phrase: "causative", tags: ["plain", "causative", "negative"] }, 
+  { from: "causative passive negative" , to: "causative negative", phrase: "active", tags: ["plain", "causative", "passive", "negative"] }, 
+  { from: "plain" , to: "passive", phrase: "passive", tags: ["plain", "passive"] }, 
+  { from: "passive negative" , to: "passive", phrase: "affirmative", tags: ["plain", "passive", "negative"] }, 
+  { from: "causative passive" , to: "passive", phrase: "non-causative", tags: ["plain", "causative", "passive"] }, 
+  { from: "passive" , to: "passive negative", phrase: "negative", tags: ["plain", "passive", "negative"] }, 
+  { from: "negative" , to: "passive negative", phrase: "passive", tags: ["plain", "passive", "negative"] }, 
+  { from: "causative passive negative" , to: "passive negative", phrase: "non-causative", tags: ["plain", "causative", "passive", "negative"] }, 
+  { from: "causative" , to: "causative passive", phrase: "passive", tags: ["plain", "causative", "passive"] }, 
+  { from: "passive" , to: "causative passive", phrase: "causative", tags: ["plain", "causative", "passive"] }, 
+  { from: "causative passive negative" , to: "causative passive", phrase: "affirmative", tags: ["plain", "causative", "passive", "negative"] }, 
+  { from: "causative negative" , to: "causative passive negative", phrase: "passive", tags: ["plain", "causative", "passive", "negative"] }, 
+  { from: "passive negative" , to: "causative passive negative", phrase: "causative", tags: ["plain", "causative", "passive", "negative"] }, 
+  { from: "causative passive" , to: "causative passive negative", phrase: "negative", tags: ["plain", "causative", "passive", "negative"] }, 
+  { from: "plain", to: "progressive", phrase: "progressive", tags: ["plain", "progressive"] }, 
+  { from: "polite progressive", to: "progressive", phrase: "plain", tags: ["plain", "polite", "progressive"] }, 
+  { from: "progressive negative", to: "progressive", phrase: "affirmative", tags: ["plain", "progressive", "negative"] }, 
+  { from: "progressive past", to: "progressive", phrase: "present", tags: ["plain", "progressive", "past"] }, 
+  { from: "progressive", to: "progressive negative", phrase: "negative", tags: ["plain", "progressive", "negative"] }, 
+  { from: "negative", to: "progressive negative", phrase: "progressive", tags: ["plain", "negative", "progressive"] }, 
+  { from: "progressive past negative", to: "progressive negative", phrase: "present", tags: ["plain", "progressive", "past", "negative"] }, 
+  { from: "polite progressive negative", to: "progressive negative", phrase: "plain", tags: ["plain", "polite", "progressive", "negative"] }, 
+  { from: "progressive", to: "polite progressive", phrase: "polite", tags: ["plain", "progressive", "polite"] }, 
   { from: "polite", to: "polite progressive", phrase: "progressive", tags: ["polite", "progressive"] }, 
   { from: "polite progressive negative", to: "polite progressive", phrase: "affirmative", tags: ["polite", "progressive", "negative"] }, 
   { from: "polite progressive past", to: "polite progressive", phrase: "present", tags: ["polite", "progressive", "past"] }, 
   { from: "polite progressive", to: "polite progressive negative", phrase: "negative", tags: ["polite", "progressive", "negative"] }, 
   { from: "polite negative", to: "polite progressive negative", phrase: "progressive", tags: ["polite", "progressive", "negative"] }, 
-  { from: "progressive negative", to: "polite progressive negative", phrase: "polite", tags: ["polite", "progressive", "negative"] }, 
+  { from: "progressive negative", to: "polite progressive negative", phrase: "polite", tags: ["plain", "polite", "progressive", "negative"] }, 
   { from: "polite progressive past negative", to: "polite progressive negative", phrase: "present", tags: ["polite", "past", "progressive", "negative"] }, 
-  { from: "past", to: "progressive past", phrase: "progressive", tags: ["past", "progressive"] }, 
-  { from: "progressive", to: "progressive past", phrase: "past", tags: ["past", "progressive"] }, 
-  { from: "progressive past negative", to: "progressive past", phrase: "affirmative", tags: ["progressive", "past", "negative"] }, 
-  { from: "polite progressive past", to: "progressive past", phrase: "plain", tags: ["polite", "progressive", "past"] }, 
-  { from: "past negative", to: "progressive past negative", phrase: "progressive", tags: ["progressive", "past", "negative"] }, 
-  { from: "progressive negative", to: "progressive past negative", phrase: "past", tags: ["progressive", "past", "negative"] }, 
-  { from: "progressive past", to: "progressive past negative", phrase: "negative", tags: ["progressive", "past", "negative"] }, 
-  { from: "polite progressive past negative", to: "progressive past negative", phrase: "plain", tags: ["polite", "progressive", "past", "negative"] }, 
+  { from: "past", to: "progressive past", phrase: "progressive", tags: ["plain", "past", "progressive"] }, 
+  { from: "progressive", to: "progressive past", phrase: "past", tags: ["plain", "past", "progressive"] }, 
+  { from: "progressive past negative", to: "progressive past", phrase: "affirmative", tags: ["plain", "progressive", "past", "negative"] }, 
+  { from: "polite progressive past", to: "progressive past", phrase: "plain", tags: ["plain", "polite", "progressive", "past"] }, 
+  { from: "past negative", to: "progressive past negative", phrase: "progressive", tags: ["plain", "progressive", "past", "negative"] }, 
+  { from: "progressive negative", to: "progressive past negative", phrase: "past", tags: ["plain", "progressive", "past", "negative"] }, 
+  { from: "progressive past", to: "progressive past negative", phrase: "negative", tags: ["plain", "progressive", "past", "negative"] }, 
+  { from: "polite progressive past negative", to: "progressive past negative", phrase: "plain", tags: ["plain", "polite", "progressive", "past", "negative"] }, 
   { from: "polite past", to: "polite progressive past", phrase: "progressive", tags: ["polite", "progressive", "past"] }, 
   { from: "polite progressive" , to: "polite progressive past", phrase: "past", tags: ["polite", "progressive", "past"] }, 
-  { from: "progressive past", to: "polite progressive past", phrase: "polite", tags: ["polite", "progressive", "past"] }, 
+  { from: "progressive past", to: "polite progressive past", phrase: "polite", tags: ["plain", "polite", "progressive", "past"] }, 
   { from: "polite progressive past negative", to: "polite progressive past", phrase: "affirmative", tags: ["polite", "progressive", "past", "negative"] }, 
   { from: "polite past negative", to: "polite progressive past negative", phrase: "progressive", tags: ["polite", "progressive", "past", "negative"] }, 
-  { from: "progressive past negative", to: "polite progressive past negative", phrase: "polite", tags: ["polite", "progressive", "past", "negative"] }, 
+  { from: "progressive past negative", to: "polite progressive past negative", phrase: "polite", tags: ["plain", "polite", "progressive", "past", "negative"] }, 
   { from: "polite progressive past", to: "polite progressive past negative", phrase: "negative", tags: ["polite", "progressive", "past", "negative"] }, 
   { from: "polite progressive negative", to: "polite progressive past negative", phrase: "past", tags: ["polite", "progressive", "past", "negative"] }, 
 ];
@@ -1124,10 +1124,8 @@ function generateQuestion() {
     var valid = true;
 
     transformation.tags.forEach(function (type) {
-      if (type != 'plain') {
-        if ($('#' + type).is(':checked') == false) {
-          valid = false;
-        }
+      if ($('#' + type).is(':checked') == false) {
+        valid = false;
       }
     });