Japanese with Manga
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Should not do with 〜てはいけない

Grammar: Prohibition » 〜てはいけない

Saying one should not perform an action is conveyed by a verb in its て form, the particle は, and the verb いけない (“wrong”, “not good”, “must not do”). Although this does not strictly prohibit an action, it gives a sense of it being wrong to do the action.

Related Grammar

  • Must not do with 〜てはだめだ
    • Used to prohibit (rather than simply discourage) an action.
  • Must not do with 〜べからず
    • An old-fashioned way to strongly forbid an action.

Examples

Don't underestimate bread-baking with 〜てはいけない

Manga panel from ご注文はうさぎですか？ showing example of Should not do with 〜てはいけない.
ご注文はうさぎですか？ » Volume 1 » Page 35

Cocoa prepares to teach her friends how to bake bread.

ココア:
「みんな　パン作(つく)りをなめちゃいけないよ！」
“Everyone, don't underestimate bread making!”

Rather than prohibiting them, this gives the sense that it would be wrong to underestimate the process of baking bread. Note that ちゃ is a casual spoken variation of ては.