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駄目だよ

You mustn’t; it’s not allowed., You have to; you must.

→ 駄目だ
phrase Uncommon #16,400
You mustn’t; it’s not allowed. A casual way to tell someone not to do something.
JA: クラスメイトの悪口を言っては駄目だよ。
EN: You mustn’t bad-mouth your classmates.

駄目な物は駄目だよ。

No means no.

窓開けたまま寝たら駄目だよ。

You shouldn't sleep with the window open.

今すぐやらなきゃ駄目だよ。

You have to do it right now.

知らない人からのメールに、むやみに返信したら駄目だよ。

You shouldn't reply indiscriminately to emails from people you don't know.

駄目、遅刻は駄目だよ。

No, being late is not okay.

彼女は禁煙しないと駄目だよ。

She needs to quit smoking.

夜遅くまで起きていては駄目だよ。

You shouldn't stay up too late.

禁煙しないと駄目だよ。

You have to quit smoking.

彼にあんな口のきき方をしては駄目だよ。

You shouldn't talk to him like that.

犬の近くに行ったら駄目だよ。

Don't go near the dog.

遅刻は駄目だよ。

Don't be late.

クラスメイトの悪口を言っては駄目だよ。

You shouldn't badmouth your classmates.

phrase Rare #38,137
You have to; you must. Used after a negative condition to say that not doing something is unacceptable. The よ adds emphasis and can make this sound like a reminder or admonition.
JA: もっと注意しなきゃ駄目だよ。
EN: You have to be more careful.

もっと自分の安全のことを考えないと駄目だよ。

You really need to think more about your own safety.

まずは誠実でいなきゃ駄目だよ。

First and foremost, you have to be honest.

もっと勉強しなくちゃ駄目だよ。

You really need to study more.

自分でやらなきゃ駄目だよ。

You have to do it yourself.

もっと注意しなきゃ駄目だよ。

You need to be more careful.