Examples
Meaning
〜てはいけない means must not do or it is not allowed to do. 〜てはいけません is the polite form often used for rules and instructions.
Formation
Use the て form of a verb and add はいけない or はいけません. For example, 入る becomes 入ってはいけない, and 話す becomes 話してはいけません.
Usage
Use this pattern for rules, restrictions, public manners, school instructions, safety warnings, and things that are forbidden.
Nuance
This pattern is stronger than a simple request not to do something. It states that the action itself is not permitted, so it can sound strict depending on the situation.
Comparison
〜ないでください asks someone not to do something. 〜てはいけない explains that the action is prohibited. 〜てはだめ is more direct and conversational.
Common Mistakes
A common mistake is using the negative form before はいけない. Say 入ってはいけません, not 入らないではいけません for this basic prohibition pattern. Also, remember that this pattern uses the て form.
Related Grammar
〜てはだめ
〜てはだめ (te wa dame) means that doing something is not okay. It is a direct and common way to prohibit an action.
〜ないでください vs 〜てはいけない
〜ないでください and 〜てはいけない both stop an action, but they are not the same. One is a request, and the other expresses prohibition.
〜なくてもいい
〜なくてもいい (nakutemo ii) means that you do not have to do something. It removes obligation and gives permission not to act.