Practice JLPTHow to tell them apart
The core contrast is permission versus obligation. 〜てもいい (with a negative becoming なくてもいい) says something is optional or acceptable to skip, while なければならない says it is required. Swapping them flips the meaning entirely: 働かなくてもいい means you may skip work, but 働かなければならない means you must work.
- てもいい Use てもいい when you are giving or asking permission and saying something is acceptable.
- なければならない Use なければならない when an action or state is required and cannot be skipped.
| Grammar point | Meaning | JLPT level |
|---|---|---|
| てもいい temoii | Don't have to | N5 |
| なければならない nakerebanaranai | Must/Have to | N4 |
てもいい temoii
Don't have to
May; It is all right if | てもいい
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A:テニスをしませんか。B:下手でもいいですか。
A: Wouldn't you like to play tennis?B: Is it alright if I'm poor at it?
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A:何か冷たいものを飲みたいんですが。 B:ビールでもいいですか。
A: I want to drink something cold. B: Will beer do?
なければならない nakerebanaranai
Must/Have to
Have to; must; need | It won’t do if someone does not take some action or if someone or something is not in some state.
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A:今日は働かなくてもいいんですか。B:いいえ、働かなければなりません。
A: Don't you have work today? B: Yes, I have work.
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あの部屋はもう少し広くなければなりません。
That room has to be a bit larger.
てもいい and なければならない are JLPT N5–N4 grammar points that Japanese learners often mix up. This page compares them side by side — meanings, usage and example sentences — so you can tell them apart.