How 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 | 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 asks or gives permission to do something. It attaches to the te-form of a verb, and is typically followed by ですか when asking (〜てもいいですか) or ですよ when granting permission.
-
何時くらいまでなら電話してもいいですか。
How late can I ring?
-
川へ行ってもいいですか。
May I go to the river?
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, that something must be done or has to be done. It attaches to the negative form of a verb with く dropped, so する becomes しなければならない and 書く becomes 書かなければならない. It is a neutral expression used in both spoken and written Japanese.
-
明日までに書かなければならないレポートがいくつかある。
I have a few essays to write by tomorrow.
-
明日までに仕事を終えてしまわなければならない。
I must have my work finished by tomorrow.
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.