Yap.Town

いけないよ

must not

→ いけない
phrase Common #8,044
must not Used after a verb in the て-form to say that something is not allowed. In なければいけない, the phrase instead means “must.” The よ adds emphasis or conveys the statement to the listener.
JA: ここで写真を撮ってはいけないよ。
EN: You must not take pictures here.

本当にタバコをやめなければいけないよ。

You really have to quit smoking.

いけないよ

You can't.

— マルコムＸ (1992)

手紙には返事を書かなきゃいけないよ。

You have to write a reply to the letter.