How to tell them apart
な attaches directly to a verb and delivers a blunt, informal command not to do something, typically from a male speaker in casual speech. てはいけない uses the て-form plus てはいけない and expresses a rule or prohibition that applies regardless of who is speaking, often in formal contexts. Swapping them feels wrong because な is too rough for posted rules like 部屋に入ってはいけない, while てはいけない is too stiff for a direct personal order like 行くな！.
- な Use な when giving a blunt, informal command to someone not to do something, typically as a male speaker.
- はいけない Use てはいけない when stating a rule or prohibition that something must not be done, in formal or neutral contexts.
| Grammar point | Meaning | JLPT level |
|---|---|---|
| な na | A command form telling someone not to do something | N5 |
| はいけない haikenai | Not allowed; must not do | N4 |
な na
A command form telling someone not to do something
な creates a strong prohibition telling the listener not to do an action. It attaches directly to the dictionary form of a verb, as in さわるな or のむな, with no further conjugation. It sounds blunt and forceful, so it is common in warnings, rules, and speech from parents or superiors, while ないでください is safer for polite requests.
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おくびにも出すな。
Don't breathe a word of it to anyone.
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あぶないから、さわるな。
It's dangerous, so don't touch it.
はいけない haikenai
Not allowed; must not do
Verb te-form + はいけない expresses prohibition and means must not do something, often for rules, manners, or moral advice. It attaches to the te-form as is, giving てはいけない after て verbs like 食べてはいけない and ではいけない after で verbs like 飲んではいけない. The form with はいけない is plain and used in casual speech or writing, while てはいけません is polite.
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口にものを入れて話をしてはいけない。
You must not speak with your mouth full.
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死んだ人たちを悪く言ってはいけない。
You mustn't speak ill of the dead.
な and はいけない 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.