The sentence in question:


While it is obvious what it means, I am wondering how precisely it is constructed grammatically.

The textbook way of expressing "must not do" using いけない is:

[て-Form] は いけない

So going by that I would construct it as follows:

出る → 出ちゃう → 出ちゃって → 出ちゃってはいけません

So I assume 「出ちゃいけません」 is simply a shortened form of this grammar.

Is that correct?

If yes, is this common?

If no, how is 「出ちゃいけません」 made up grammatically?


No, short for 出ちゃってはいけません would be 出ちゃっちゃいけません. ~ては itself contracts to ~ちゃ (compare ではない and じゃない), so 出ちゃいけません is a contracted form of 出てはいけません. This kind of contraction is extremely common.

  • Oh, right, makes sense. I completely forgot about ては→ちゃ. – user40476 Nov 27 '20 at 18:49

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