Must not do with 〜てはだめだ
Grammar: Prohibition » 〜てはだめだ
〜てはだめだ directly says that an action is not allowed or will not do. In speech, ては often contracts to ちゃ.
Usage Notes
〜てはだめだ is a direct way to say an action is not allowed or is no good. It can sound more immediate and conversational than 〜てはいけない.
In casual speech, ては contracts to ちゃ, as in しちゃだめ. 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. 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. During breakfast, Cocoa sees Chino avoiding the vegetable on her plate.Related Grammar
Examples
Don't be found with 〜てはだめだ
Don't grow bigger with 〜てはだめだ
Don't leave your vegetables 〜てはだめだ
