In short

Japanese has no word for “must”. It says the same thing with a double negative built on the ない-form: 行かなければなりません ikanakereba narimasen, literally “if I don’t go, it won’t do” - so, “I have to go”. The opposite, “you don’t have to”, is 〜なくてもいいです nakute mo ii desu. For advice, use the た-form + ほうがいいです: 休んだほうがいいです yasunda hō ga ii desu, “you’d better rest”.

The rule

Step 1 - Start from the ない-form, not the dictionary form. Everything in the obligation family is stacked on the plain negative you learned in Plain forms. Get 行かない ikanai first, then chop the ない off and add one of two connectors:

• ない -> なければ (drop い, add ければ): 行かなければ ikanakereba
• ない -> なくては (the て-form of ない, plus the particle は): 行かなくては ikanakute wa

Both mean “if not …”. This is the ば-conditional and the て-form doing ordinary jobs; the full ば system waits for Conditionals 〜ば and 〜なら.

Step 2 - Finish with a negative “no good”. Add なりません narimasen (“it won’t do”) or いけません ikemasen (“it won’t go”). Four combinations, all meaning “must”:

〜なければなりません / 〜なければいけません / 〜なくてはなりません / 〜なくてはいけません

The negative at the end is compulsory - it is the second half of the double negative. There is no ×行かなければなります.

Step 3 - Pick the right one. The differences are register and flavour, not meaning:

• ならない is more written and impersonal - rules, laws, forms, announcements.
• いけない is more spoken and more situational - this particular obligation, right now.
• なければ feels slightly more formal and general; なくては slightly more colloquial and personal.

So 税金を払わなければなりません (Zeikin o harawanakereba narimasen, “taxes must be paid”) reads like a regulation, while もう帰らなくてはいけません (Mō kaeranakute wa ikemasen, “I have to head home now”) sounds like a person in a room.

Step 4 - Learn the contractions for listening. In speech the long endings collapse, and the “no good” half is usually dropped entirely - the sentence just trails off and the listener supplies the rest.

• なければ -> なきゃ nakya: 行かなきゃ。 Ikanakya.
• なくては -> なくちゃ nakucha: 行かなくちゃ。 Ikanakucha.
• 〜ないと nai to: 行かないと。 Ikanai to. - the commonest of the three in everyday talk.

These are plain-style, so do not attach です／ます to them. The whole contraction system is Casual style and contracted forms; here just recognise them.

Step 5 - “Don’t have to” is a different pattern. Take the なくて half, add も (“even”) and いいです: 〜なくてもいいです nakute mo ii desu, “it’s fine even if you don’t”. It is the negative twin of 〜てもいいです from Permission, prohibition and commands. A warmer variant is 〜なくても大丈夫です nakute mo daijōbu desu (“you’re fine without doing it”).

Watch the particle: も = “even if not” -> permission; は = “if not” -> obligation. 行かなくてもいいです is “you needn’t go”; 行かなくてはいけません is “you must go”.

Step 6 - Necessity with 必要. 必要 hitsuyō is a noun (“necessity”) and a な-adjective (“necessary”) - never a verb, so it takes noun-style grammar:

• Noun + が必要です ga hitsuyō desu - “X is needed”: ビザが必要です。
• V-plain (dictionary) + 必要があります hitsuyō ga arimasu - “there is a need to …”.
• Negative: 必要はありません hitsuyō wa arimasen - “there is no need to”. The particle usually flips to は under negation.

必要 states a fact about the situation; 〜なければなりません puts the duty on a person. A form saying 記入する必要があります (“it is necessary to fill this in”) is describing the procedure, not ordering you around.

Step 7 - Advice: ほうがいいです, and mind the tense. The pattern is V-た + ほうがいいです for “you’d better do it” and V-ない + ほうがいいです for “you’d better not”. The asymmetry is real and must be memorised: the positive side normally uses the past form, the negative side uses the plain non-past negative. (The dictionary form before ほうがいい does occur, but it reads as a general comparison rather than advice to this person.)

• 行ったほうがいいです itta hō ga ii desu - “you should go”
• 行かないほうがいいです ikanai hō ga ii desu - “you shouldn’t go”

ほうが is literally “the side of”, so ほうがいい weighs two courses and picks one: it implies the alternative is the worse option, which makes it real advice rather than a passing idea. Softened with よ (〜たほうがいいですよ) it is everyday friendly advice. When you only want to float a suggestion without weighing it against an alternative, use V-plain + といいですよ to ii desu yo (“it’d be good if you …”).

Step 8 - Softer still: suggestions. 〜たらどうですか tara dō desu ka (“how about …?”) and 〜たらいいと思います tara ii to omoimasu (“I think it’d be good to …”) both sit on the たら-conditional from Conditionals 〜たら and 〜と and are the gentlest way to steer someone.

A note on 〜べきだ. V-plain + べきです beki desu is the moral “ought to” - what is right, not what the rules force. It is formal and slightly preachy in speech, and する has the special form すべきだ beside するべきだ. Recognise it; it belongs to Written and formal register.

Forms & patterns

Building the obligation stem. Everything starts from the plain negative.

The four endings and their contractions.

From softest to hardest. Same verb 休む yasumu (“rest, take time off”), six strengths - plus 〜なくてもいいです, which is not a weak point on the scale but the zero point: it cancels the obligation instead of grading it.

Examples

Common mistakes

• 行くなければなりません。 (Iku nakereba narimasen.) -> 行かなければなりません。 (Ikanakereba narimasen.) - Build it from the ない-form 行かない, never from the dictionary form. Find 行かない first, then swap ない for なければ.
• 行かなくてもいけません。 (Ikanakute mo ikemasen.) -> 行かなくてはいけません。 (Ikanakute wa ikemasen.) - One particle decides the meaning: は gives obligation, も gives permission. 〜なくてもいいです is “needn’t”; there is no obligation pattern ×なくてもいけません.
• きなければなりません。 (Kinakereba narimasen.) -> こなければなりません。 (Konakereba narimasen.) - 来る is irregular: its ない-form is 来ない konai, so the whole obligation chain keeps the reading ko-, not ki-.
• メールを送るほうがいいです。 (Mēru o okuru hō ga ii desu.) -> メールを送ったほうがいいです。 (Mēru o okutta hō ga ii desu.) - Positive advice takes the た-form. The dictionary form before ほうが sounds like a bare comparison, not advice.
• 行かなかったほうがいいです。 (Ikanakatta hō ga ii desu.) -> 行かないほうがいいです。 (Ikanai hō ga ii desu.) - Negative advice takes plain ない, never なかった. Only the positive side goes past.

Tips

• Count the negatives. Every obligation sentence here contains exactly two: one on the verb (なければ／なくては) and one at the end (なりません／いけません). If you can only find one, the sentence is broken. Learners typically lose the second half and say ×行かなければです.
• Say the whole ending as one chunk. Drill -nakereba narimasen and -nakute wa ikemasen as fixed blocks attached to a stem, exactly like a suffix. Native speed makes them sound like one ending anyway, and in speech they shrink to なきゃ／なくちゃ／ないと - which is what you will actually hear.
• ほうがいい is a comparison, not a neutral “should”. It picks your course over the alternative, so it lands as advice the speaker means, not as a passing thought. For a light suggestion - a restaurant, a film - 〜といいですよ or 〜たらどうですか sit more softly.
• 必要 is a noun／な-adjective, so watch the particles. It takes が in the affirmative (時間が必要です) and usually は in the negative (その必要はありません, “no need for that”). Do not treat it as a verb: there is no ×必要します.