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).

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