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, 〜なくちゃ).
- 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 a unique 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 this chapter, we will master the nuances of “must,” “don’t have to,” and the strong “should” used for social and moral expectations.
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.
毎日、薬を飲まなければなりません。
mainichi, kusuri o nomanakereba narimasen.
I must take medicine every day.
試験の前に、もっと勉強しなければなりません。
shiken no mae ni, motto benkyou shinakereba narimasen.
I must study more before the exam.
Casual Variations
In spoken Japanese, 〜なければなりません is often shortened for speed and natural flow.
- 〜なきゃ(いけない/だめ): The most common spoken casual form.
- 〜なくちゃ(いけない/だめ): Similar to なきゃ, but slightly more colloquial.
- 〜ないと(いけない/だめ): Meaning “If I don’t [do X], then [it’s bad].“
もう遅いから、帰らなきゃ。
mou osoi kara, kaeranakya.
Because it's late already, I gotta go home.
あっ、レポートを出さなきゃいけないんだ。
a, repooto o dasanakya ikenai n da.
Oh, I have to submit the report.
明日、早く起きないとだめだよ。
ashita, hayaku okinai to dame da yo.
You must wake up early tomorrow, okay?
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.
嫌いな物は、食べなくてもいいですよ。
kirai na mono wa, tabenakute mo ii desu yo.
You don't have to eat things you dislike.
急がなくてもいいです。時間は十分にあります。
isoganakute mo ii desu. jikan wa juubun ni arimasu.
You don't have to hurry. There is plenty of time.
3. Strong Advice: “Should” (〜べき)
When you want to express that something is logically, societally, or morally the right thing to do, use 〜べき. This is much stronger than the advice form 〜ほうがいい.
Formation: Attach べき directly to the Dictionary Form of a verb.
(Special Case: 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.
お年寄りには親切にするべきです。
otoshiyori ni wa shinsetsu ni suru beki desu.
One should be kind to the elderly.
To say “should not do”, keep the verb in the dictionary form and negate the べき: 〜べきではない.
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.
Cultural Insight: Responsibility and “Giri”
In Japanese society, obligation is often framed through Giri (義理 - social obligation) and Sekinin (責任 - responsibility). While English speakers might say “I have to” as a personal requirement, Japanese speakers often use 〜なければならない to express a duty to the group (family, company, or society).
Failing to meet an obligation can cause Meiwaku (迷惑 - trouble/inconvenience) to others. This is why 〜べき is so strong; it implies that failing to do the action violates the harmony of the group.
Contextual Dialogues
Dialogue 1: At the Doctor’s Office
毎食後、この薬を飲まなければなりません。
Maishokugo, kono kusuri o nomanakereba narimasen.
You must take this medicine after every meal.
来週も来なければなりませんか。
Raishuu mo konakereba narimasen ka.
Do I have to come back next week too?
具合がよくなったら、来なくてもいいですよ。
Guai ga yokunattara, konakute mo ii desu yo.
If you feel better, you don't have to come.
わかりました。健康に気をつけなきゃ！
Wakarimashita. Kenkou ni ki o tsukenakya!
I see. I've gotta be careful about my health.
Dialogue 2: Late Night at the Office
明日までにこのレポートを終わらせなければならないんです。
Ashita made ni kono repooto o owarasenakereba narana n desu.
I must finish this report by tomorrow.
無理をするべきではありませんよ。もっと寝るべきです。
Muri o suru beki dewa arimasen yo. Motto neru beki desu.
You shouldn't work too hard. You should sleep more.
わかっていますが、やらなきゃいけないんです。
Wakatte imasu ga, yaranakya ikenai n desu.
I know, but I've gotta do it.
そうですか。大変なら、一人で全部やらなくてもいいですよ。手伝いましょうか。
Sou desu ka. Taihen nara, hitori de zenbu yaranakute mo ii desu yo. Tetsudaimashou ka.
Well, if it's too difficult, you don't have to do it all alone.
Chapter Summary
- 1Strong obligation (must): 〜なければならない (Formal) / 〜なきゃ (Casual).
- 2Lack of obligation (don't have to): 〜なくてもいい.
- 3Moral/Social duty (should): Dictionary form + べき.
- 4Strong prohibition of advice (should not): Dictionary form + べきではない.
- 5Japanese obligation often reflects a sense of social responsibility toward the group.
Knowledge Check
Quiz
Which of the following means 'I must go'?