Japanese with Manga
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Must not do with 〜てはだめだ

Grammar: Prohibition » 〜てはだめだ

Saying one must not perform an action is conveyed by a verb in its て form, the particle は, and the adjective だめ (“cannot”, “must not”, “not allowed”), ending in だ.

Related Grammar

  • Should not do with 〜てはいけない
    • Used to discourage (rather than prohibit) an action.
  • Must not do with 〜べからず
    • An old-fashioned way to strongly forbid an action.

Examples

Don't be found with 〜てはだめだ

Manga panel from ご注文はうさぎですか？ showing example of Must not do with 〜てはだめだ.
ご注文はうさぎですか？ » Volume 1 » Page 72

While telling ghost stories at night, Rize relays the tale from her childhood, of a frightened servant walking home one night. Rize says she herself was the culprit, practicing sneaking around and crawling through the bushes.

「バラしちゃだめじゃん！！」
“Don't be found!!”

Note that じゃん is a short form of じゃない, the negative of だ.

Don't grow bigger with 〜てはだめだ

Manga panel from ご注文はうさぎですか？ showing example of Must not do with 〜てはだめだ.
ご注文はうさぎですか？ » Volume 1 » Page 82

On the topic of Chino outgrowing Cocoa, Cocoa is fine with it until she learns Chino won’t be like a doting big sister to her.

ココア:
「チノちゃんは大(おお)きくなっちゃだめ！」
“You mustn't become bigger.”

Note that だ has been dropped from the expression here.