Grammar JLPT Level N5 - Lesson 18

Structure of ～てはいけない:

Verb［て］+ は + いけない

Details about ～てはいけない:

てはいけない is used in Japanese to express that (A) 'cannot', or 'must not' be done. It is the combination of a verb in て form, the は particle, and the negative potential form of 行(い)く (to be able to go). In this way, it literally means '(A) cannot/is not able to go'.

タクシーの扉(とびら)を自分(じぶん)で開(あ)けてはいけない。
You must not open the taxi door by yourself.
子供(こども)の前(まえ)でタバコを吸(す)ってはいけません。
You must not smoke cigarettes in front of children.

Although は can be dropped from many phrases and expressions, the は in this particular grammar point is usually not. This is because いけない would then become the second action in a sequence of events (the usual function of て form). However, to make this more casual, ては is often changed to ちゃ (ては becomes ちゃ), or じゃ (では becomes じゃ), as this is easier to say.

危(あぶ)ない場所(ばしょ)に行(い)っちゃいけない。
You must not go to dangerous places.
旗(はた)を踏(ふ)んじゃいけないよ。
You must not step on a flag.

～てはいけない

Usage Notes for ～てはいけない:

Apart from いけない; ならない and だめ are also frequently used to get across the same meaning. However, they are used in different situations.
いけない - Fairly neutral. Used in both spoken and written language.
ならない - Fairly formal. Used mostly in written language.
だめ - Very casual. Used almost exclusively in spoken language.