Translated examples
ここに入ってはだめです。
You must not enter here.
友だちの物を勝手に使ってはだめです。
You must not use your friend's things without asking.
約束を忘れてはだめです。
You must not forget your promise.
道で遊んではだめです。
You must not play in the street.
Meaning
〜てはだめ means must not do or it is bad to do. It tells the listener that an action should not be done.
Formation
Use the て form of a verb and add はだめ. In polite speech, use 〜てはだめです. In casual speech, 〜ちゃだめ is common.
Usage
Use this pattern when giving direct warnings, correcting behavior, or telling someone that an action is not acceptable. It is common in family, school, and casual daily situations.
Nuance
This pattern is more direct and conversational than 〜てはいけません. It can sound strict, especially when spoken to adults, so politeness and relationship matter.
Comparison
〜てはいけません sounds more rule-like and formal. 〜てはだめ sounds more direct, personal, and conversational. Both express prohibition.
Common Mistakes
A common mistake is using だめ for every prohibition situation. In formal signs or official rules, 〜てはいけません is often more appropriate. Also, do not forget to use the て form before はだめ.
Related Grammar
〜てはいけない・〜てはいけません
〜てはいけない and 〜てはいけません express prohibition. They mean that doing something is not allowed.
〜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.