# Obligation

## が必要・必要がある

Means "needed" or "necessary".
が{必要|ひつよう} is used mainly with nouns (can be used with verbs by appending こと before).
{必要|ひつよう}がある is used with verbs.

> [!info] Example
> {今時|いまどき}、{携帯電話|けいたいでんわ}==が必要です==。
> Nowadays, a cell phone is necessary.
> 
> 今日はパン{屋|や}に行く==必要がない==。
> No need to go to the bakery today.

## わけにはいかない

Used with verbs for a meaning of "cannot do" (which cannot be your inability, but an external factor).
Used with verbs in negative form (ない) for a meaning of "cannot be avoided".
Can be used as わけはいかない or わけにはいきません.

> [!info] Example
> {今日|きょう}の{弟|おとうと}の{誕生日|たんじょうび}に行か==ないわけにはいかない==だから、{飲|の}み{会|かい}に行かない。
> There's no way I don't go to my younger brother's birthday today, so I won't go to the afterwork.
> 
> {本当|ほんとう}に飲み会に行く==わけにはいかない==よ。
> There's really no way I can go to the afterwork.

## ずにはいられない

Means "can't help but do something".
Made with a verb in the negative form (ない) without the ない ending and adding ずにはいられない. 

> [!info] Example
> {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

Only used on casual spoken conversations.
This form is used with the negative form of a verb (ない), and adding なくちゃ and optionally one of the following ending: だめ, いけない, いけません, ならない, なりません.

なくちゃ is the shortened form of なくては.
なきゃ is the shortened form of なければ.

> [!info] Example
> {歯|は}を{磨|みが}か==なくちゃいけない==。
> I must brush my teeth.

### なくては「いけない・ならない」

Also formed with the negative form of a verb.

> [!info] Example
> 今{遅|おそ}いね。{寝|ね}に行か==なくてはいけない==。
> It's late now. I have to go to sleep.

### なければ「いけない・ならない」

Also formed with the negative form of a verb.

> [!info] Example
> {事務所|じむしょ}では{静|しず}かにし==なければならない==。
> You have to be quiet in the office.

### ないといけない

Also formed with the negative form of a verb.
In casual contexts, can be shortened as ないと.

> [!info] Example
> 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

Means "should do", "must do".
Used with verbs.
する can be used as するべき or すべき.

> [!info] Example
> {来週|らいしゅう}{試験|しけん}があるので{勉強|べんきょう}す==べきです==。
> There is an exam next week, so I should study.

## てからでないと

Means "must first do". The second part becomes possible after the first one.
Used with a verb in て form, with からでないと or からでないければ.

> [!info] Example
> {払|はら}==ってからでないと=={入|はい}るができること。
> You can enter only after paying.


🛠🛠🛠🛠
Todo: new notions!

### Comparing いけない and ならない

* ==いけない== is the standardly used form. It implies a subjective view, that it would be better to do something, to avoid negative consequences (eg: brushing one's teeth, eating well, etc).
* ==ならない== is usually more formal or in written form. It implies a is more objective view, something that has to be done because there's no other way (eg: respecting the law, paying taxes, etc).

### Comparing なくては, なければ and ないと

なくては and なければ are generally interchangeable in speech. However:
* ==なくては== is generally used when there is _no penalty_ for not complying.
* ==なければ== is generally used when there _is a penalty_ for not complying.
* ==ないと== is lighter and does not imply that there can or cannot be penalty. Its construction is based on the [[Conditional#と]] particle, and should be based on knowledge or experience.

## Must not

#### 「ちゃ・じゃ」「だめ・いけない・いけません」

Only used on casual spoken conversations. いけません is a bit more formal.
This form is used with a V-て, depending on the ending:
* with て, ちゃ is used.
* with で, じゃ is used.

> [!info] Examples
> その{花|はな}を食べ==ちゃだめ==。
> Don't eat those flowers.
> 
> {花瓶|か|びん}の水を{飲|の}ん==じゃいけない==。
> Don't drink the water of the flower vase.

### てはいけない

Used in a more formal or written manner. Used with a V-て.

> [!info] Example
> {鉄道|てつ|どう}に{歩|ある}い==てはいけません==。
> Do not walk on the railroad.

### なくてもいい

Means "don't have to". Used with a verb in the negative form (ない), appending なくてもいい.
Can also be used as なくていい.

> [!info] Example
> 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

The contrary of べきだ is べきではない・べきじゃない.

> [!info] Example
> {泥|どろ}を食べる==べきではない==。
> You should not eat mud.








# TODO

## ないと





must do; unless/if you don't ~

https://jlptsensei.com/learn-japanese-grammar/%e3%81%aa%e3%81%84%e3%81%a8-naito-meaning/
https://bunpro.jp/grammar_points/%E3%81%AA%E3%81%84%E3%81%A8