Update Obligation.md

🔰 Grammar/Obligation.md

@@ -35,6 +35,43 @@ Made with a verb in the negative form (ない) without the ない ending and add
 > {悲|かな}しいのでアイスクリーム食べ==ずには入れれない==。
 > I'm sad so I can't help but eat ice cream.
 
+## こと as "should do"
+
+<div class="usage">
+<div class="left">
+	<p><span class="box">V (dictionary)</span></p>
+	<p><span class="box">V (ない)</span></p>
+</div>
+	<p class="right">+ こと</p>
+</div>
+
+Means "should do". Mostly used to set rules.
+
+> [!info] Example
+> ここに{車|くるま}を{止|と}めない==こと==です。
+> Don't park your car here.
+
+## ことはない
+
+This has two meanings:
+* it is not necessary to
+* it never happens
+
+Used with a verb in the dictionary form.
+
+> [!info] Example
+> {走|はし}る==ことはない==。まだ{時間|じかん}がある。
+> There's no need to run. There's still time.
+
+## ことになっている
+
+This form is used to express rules or expectations.
+Used with dictionary (る) and negative (ない) form of verbs.
+
+> [!info] Examples
+> {指|ゆび}で食べない==ことになっている==。
+> You're not supposed to eat with your fingers.
+
 ## Must
 
 ### なくちゃ・なきゃ
@@ -74,7 +111,7 @@ In casual contexts, can be shortened as ないと.
 > 今日は{勉強|べんきょう}し==ないといけない==。
 > I have to study today.
 
-## べきだ
+### べきだ
 
 Means "should do", "must do".
 Used with verbs.
@@ -142,7 +179,7 @@ Can also be used as なくていい.
 > 今日はレストランで{手伝|て|つ}わ==なくてもいい==。
 > You don't have to help out at the restaurant today (it's okay if you don't help at the restaurant today).
 
-## べきではない・べきじゃない
+### べきではない・べきじゃない
 
 The contrary of べきだ is べきではない・べきじゃない.