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.