～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'.

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.

Caution

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.