jpg-content/1- Grammar/Must (not).md

# Must & must Not

## 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.

## 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.

### 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).