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LEARN + JAPANESE / 17 of 186 JLPT N5

Go Ahead, or Don't You Dare てもいい・てはいけない

てもいい attaches to a verb’s て-form to mean “may, it’s okay to.” てもかまわない also grants permission, with a somewhat more formal sense of “I don’t mind if you do.” てはいけない attaches the same way to mean “must not, it’s forbidden.” Same て-form both directions, opposite permission.

てはならない is a more formal or rule-like prohibition than てはいけない: ここに駐車してはならない, “parking here is prohibited.” It is common in written rules, signs, and formal statements.

Hint · 힌트
てもいい literally reads “even if [you] do this, it’s good,” も carrying its “even” meaning from Anyone, Anytime, Anywhere straight into this construction. Asking permission with か on the end is the standard, polite way to check before doing something.
Hint · 힌트
てはいけない is a flat prohibition, stronger than simply saying you don’t want to do something. なくてはいけない (the negative te-form plus はいけない) flips the meaning to obligation instead, “not doing this isn’t allowed,” which lands as “you have to.” Negating a negative to state a requirement is a pattern that shows up constantly in Japanese.

In blunt casual speech, a dictionary-form verb + んじゃない can also mean “don’t do that.” It sounds like a direct command and is most common when a parent, teacher, or close friend is warning someone. Use ないでください or てはいけません when you need a more polite tone.

In Real Life
This is a firm warning, not a neutral rule. Between adults it can sound angry or patronizing, so soften it to 近づかないでください when speaking politely.

Vocabulary

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