ちゃいけない・じゃいけない

About

～てはいけない ("must not do X") can be made more casual by substituting ては for ちゃ or じゃ to form ～ちゃいけない.

E.g. 食たべる → 食たべて → 食たべちゃいけない

When a verb's て form uses で, ちゃいけない will become じゃいけない instead.

E.g. 飲のむ → 飲のんで → 飲のんじゃいけない

If combined with a negative て form verb, ～ちゃいけない forms a double negative and expresses "must" instead.

E.g. 飲のまなくちゃいけない "must drink"

ちゃいけない is quite casual and should be avoided in polite/formal situations.

Summary

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  ～ちゃいけない・～じゃいけない can be used for a casual "must not do X"

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  To use ～ちゃいけない attach it to a て form verb after removing て

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  ～じゃいけない is used when the て form verb ends with で

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  ちゃいけない can be combined with negative て form verbs to express "must"