〜ないでください vs 〜てはいけない

Meaning

〜ないでください means please do not do something. 〜てはいけない means that doing something is not allowed.

Formation

For 〜ないでください, use the ない form of the verb and add でください. For 〜てはいけない, use the て form and add はいけない or はいけません.

Usage

Use 〜ないでください when politely asking someone not to act. Use 〜てはいけない when explaining a rule, restriction, or prohibition.

Nuance

〜ないでください focuses on the speaker's request. 〜てはいけない focuses on the rule or the fact that the action is not permitted. This makes 〜てはいけない stronger.

Comparison

写真を撮らないでください asks someone not to take photos. 写真を撮ってはいけません says taking photos is not allowed. The second sentence is more rule-like.

Common Mistakes

A common mistake is using the two patterns as if they were identical. In a personal request, 〜ないでください may be better. In a public rule or warning, 〜てはいけません is usually clearer.