How to tell them apart
The core contrast is permission versus obligation. てもいい says doing something is acceptable, as in 働いてもいい, while なければならない says doing something is required, as in 働かなければならない. The negative 働かなくてもいい means you may skip work, which is the opposite of the obligation form.
- てもいい Use てもいい to give or ask permission and say doing something is acceptable.
- なければならない Use なければならない to say an action is required and must be done.
| Grammar point | Meaning | JLPT level |
|---|---|---|
| てもいい temoii | May I; it's okay to; permission to do | N5 |
| なければならない nakerebanaranai | obligation to do something; required to | N4 |
てもいい temoii
May I; it's okay to; permission to do
てもいい expresses permission, meaning it is okay to do something or asking if you may do it. It attaches to the te-form of a verb, as in 食べてもいい or 行ってもいい, with もよい as a slightly more formal equivalent of もいい. In polite requests it is common with ですか, as in 入ってもいいですか, to ask whether an action is allowed.
-
何かを食べてもいいんですか。
May I eat something?
-
これ食べてもいい？
Can I eat this?
なければならない nakerebanaranai
obligation to do something; required to
なければならない expresses obligation or necessity, meaning someone has to do something or is required to do it. It attaches to the nai-form of a verb with ない removed, as in 行かない to 行かなければならない, with the polite form なければなりません. Although it contains a negative conditional, the meaning is affirmative, and casual speech often shortens it to なきゃ or なくちゃ.
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私は明日の朝早くここを出発しなければなりません。
I have got to leave here early tomorrow morning.
-
あなたはすぐに行かなければならない。
You must go at once.
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 on meanings, usage and example sentences, so you can tell them apart.