The most standard way of saying “to have to” in Japanese is to say, “if you don’t do (x) then things can’t go well” or “if you don’t do (x) things won’t become”. And we actually have three ways of constructing this logic, each of which follows a double negative pattern to create the expression of “must do”.
【Verb: Neg. conditional ～ば】【いけない or ならない】
【Verb: Negative】といけない
【Verb: Negative te-form (～なくて)】は【いけない or ならない】
掃除をしなければならない。
I have to tidy up.
掃除をしないといけない。
I have to tidy up.
彼女に電話しなくてはならない。
I need to phone my girlfriend.
ごめん！今日は無理ね。しなければいけない用事があるのよ。
Sorry! Today’s impossible. I’ve got a task I need to do.
Note the effeminate omission of the auxiliary verb and のよ at the end.
今朝は、銀行に行かなければなりませんでした。
This morning I had to go to the bank.
This can seem like a tongue-twister at first. In casual conversation there are a couple of ways to abbreviate the phrase. Firstly, the conditional form なければ can be shortened to なきゃ, and なくては can be shortened to なくちゃ.
プレゼンがあるので、準備しなくちゃいけない。
I have a presentation so I need to prepare.
明日は８時から電話会議があるから寝なきゃいけない。
Tomorrow I have a conference call from 8 o’clock so I’ve got to get to sleep.
Alternatively, you can use a more simple word like だめ in place of “to go” or “to become”.
試験がもうすぐだから勉強しなきゃだめだよ。
The exam is right around the corner so you’ve got to study.
Or simply drop the final verb altogether.
やらなきゃ。
I have to do it.
友達だから本当のことを言わなきゃ。
They’re your friend so you need to tell them the truth.
社員は入社する前に健康診断を受けなくてはいけません。
Employees must take a health check-up before joining the company.
One other way to express requirement is to modify the adverbial noun 必要 (necessity).
留学生は区役所で登録する必要があります。
Foreign students need to register at the ward office.
Must Not (〜てはいけない)
To form the negative expression (must not), we take the standard te-form structure and pair it with the negative verbs.
【Verb: te-form】は【いけない or ならない】
Typically いけない is used.
試験中に携帯電話を使ってはいけません。
During the exam you are not permitted to use a mobile phone.
The kanji 中 means “middle” and can often be added to other words to mean “during”.
電車の中では携帯で話してはいけない。
Speaking on the phone is prohibited inside the train.
それは決して忘れてはいけないことだよ。
That is something you must absolutely not forget.
Don’t Have To (〜なくてもいい)
Crucial JLPT Note: English speakers often confuse “must not” with “don’t have to.” In Japanese, these are completely different grammar points, and exams love to test this difference. To say you do not have to do something, you use the negative te-form plus もいい (it is okay if you don’t).
明日は休みだから、早く起きなくてもいい。
Tomorrow is my day off, so I don’t have to wake up early.