Expressing Obligation (Must, Must Not, Should)
Learning Objectives
- Understand the double-negative logic of 'Must do' (〜なければならない).
- Learn the casual forms of obligation (〜なきゃ).
- Express 'don't have to' using the negative te-form (〜なくてもいい).
- Give strong moral/logical advice using 〜べき.
Telling someone what they MUST do in Japanese involves some interesting logic: you say, “If you don’t do it, it won’t be okay.” This double-negative structure is the standard way to express obligation in Japanese. Let’s learn how to navigate “must,” “don’t have to,” and “should.”
1. Obligation: “Must Do”
There are several ways to say “must do,” but they all follow the same grammatical formula: take the Nai-form of a verb, drop the final い, and add a conditional suffix (ければ or きゃ) followed by なりません or だめ.
The most formal and standard written form is 〜なければなりません. (Literally: If you do not do [verb], it will not become [okay]).
明日は早く起きなければなりません。
ashita wa hayaku okinakereba narimasen.
Tomorrow I must wake up early.
薬を飲まなければなりません。
kusuri o nomanakereba narimasen.
I must take medicine.
Casual Variations
In spoken Japanese, 〜なければなりません is far too long and stiff. You will very commonly hear these shortened versions:
- 〜なきゃ(いけない/だめ): The most common spoken casual form. (行く ➔ 行かなきゃ - I gotta go).
- 〜なくちゃ(いけない/だめ): Similar to なきゃ, but slightly cuter/more colloquial. (行く ➔ 行かなくちゃ).
- 〜ないと(いけない/だめ): Very common. Meaning “If you don’t…” (行く ➔ 行かないと - I must go).
もう遅いから、帰らなきゃ。
mou osoi kara, kaeranakya.
Because it's late already, I gotta go home.
2. Lack of Obligation: “Don’t Have To”
To say that an action is unnecessary (“you don’t have to do it”), you use the negative te-form (〜なくて) followed by もいいです.
(Literally: Even if you do not do [verb], it is fine).
明日は休みだから、早く起きなくてもいいです。
ashita wa yasumi dakara, hayaku okinakute mo ii desu.
Tomorrow is a day off, so I don't have to wake up early.
嫌なら、食べなくてもいいですよ。
iya nara, tabenakute mo ii desu yo.
If you dislike it, you don't have to eat it.
3. Strong Advice: “Should” (〜べき)
When you want to express that something is logically, societally, or morally the right thing to do, you use 〜べき.
This is much stronger than 〜ほうがいい (It’s better if you…). 〜べき implies that failing to do it violates common sense or rules.
Formation: Attach べき directly to the Dictionary Form of a verb.
(Exception: する can become either するべき or すべき).
約束は守るべきだ。
yakusoku wa mamoru beki da.
Promises should be kept.
学生はもっと勉強すべきです。
gakusei wa motto benkyou subeki desu.
Students ought to study more.
To say “should not do”, keep the verb in the dictionary form and negate the べき: 〜べきではない (Should not be that you…).
そんなひどいことを言うべきではありません。
sonna hidoi koto o iu beki dewa arimasen.
You should not say such terrible things.
Contextual Dialogue
Let’s see how Tanaka and Suzuki talk about rules, stress, and obligations.
田中さん、疲れているようですね。
Tanaka-san, tsukarete iru you desu ne.
Tanaka-san, you look tired.
ええ。最近、毎日夜十時まで働かなければならないんです。
Ee. Saikin, mainichi yoru juu-ji made hatarakanakereba narana n desu.
Yes. Recently, I have had to work until 10 PM every day.
それは体に悪いですよ。もっと休むべきです。
Sore wa karada ni warui desu yo. Motto yasumu beki desu.
That is bad for your body. You should rest more.
わかっているんですが、今大きなプロジェクトがあって……。
Wakatte iru n desu ga, ima ookina purojekuto ga atte......
I know, but there is a big project going on right now.
それでも、無理をするべきではありません。明日は来なくてもいいんでしょう？
Soredemo, muri o suru beki dewa arimasen. Ashita wa konakute mo ii n deshou?
Even so, you should not push yourself too hard. You don't have to come in tomorrow, right?
明日は日曜日だから、働かなくてもいいです。たくさん寝なきゃ！
Ashita wa nichiyoubi dakara, hatarakanakute mo ii desu. Takusan nenakya!
Tomorrow is Sunday, so I don't have to work. I’ve gotta sleep a lot!
Chapter Summary
- 1Must do: 〜なければならない (Lit: If not done, unacceptable).
- 2Spoken Must do: 〜なきゃ / 〜なくちゃ / 〜ないと.
- 3Don't have to: 〜なくてもいい (Lit: Even if not done, good).
- 4Should (Strong Advice): Dictionary form + べき (Lit: Ought to).
- 5Should Not: Dictionary form + べきではない.
Knowledge Check
Test your understanding of Obligations!
Quiz
What is the literal translation of '行かなければならない'?