This means you must not do something. It is a strong way to say "no" to an action.
Verb [て-form] + は + だめPrimary
This means you must not do something. It is a strong way to say "no" to an action.
夜遅くまでゲームをしてはだめよ。
You must not play games until late at night.
ここは神聖な場所だから、大声で話してはだめだ。
This is a sacred place, so you must not speak loudly.
子供の時、暗くなってから外で遊んではだめだとよく言われた。
When I was a child, I was often told that I was not allowed to play outside after it got dark.
まだ医者の許可がないから、勝手に退院してはだめですよ。
You don't have the doctor's permission yet, so you absolutely cannot discharge yourself from the hospital.
こんな大事な試合で、最初から諦めてはだめだ。
In an important match like this, you can't give up from the very beginning.
This phrase shows your strong personal opinion. It is like a parent telling a child 'no'. You should not use it with people older than you. You should not use it with strangers.
This is different from "~te wa ikemasen". That phrase is for general rules. It is also for formal places or when a boss speaks. This phrase is more casual. You use it with family or close friends. Parents often use it with children.
Don't use this when you want to say "you must do something." For example, "食べなくてはだめ" means "you must eat."
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Less formal alternatives(2)
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