Meaning:

“You must not…” / “It is not allowed to…”

Used to express prohibition or strong rules. It is the opposite of ～てもいい (which expresses permission).

Structure:

Verb (て-form) + はいけない

💡 Can also be shortened in casual speech to:

• ～ちゃいけない (for て-form of う/つ/る verbs)

• ～じゃいけない (for で-form verbs)

Examples:

1. ここでたばこをすってはいけません。
   → You must not smoke here.

2. 授業中にスマホを使ってはいけない。
   → You are not allowed to use a smartphone during class.

3. 他人の物を勝手に使ってはいけません。
   → You must not use other people’s things without permission.