Cha ちゃ

Informal equivalent of te wa or tara (usually used by women)

Meaning: if, when

It is used after the stem of te-form of verbs whose dictionary form ends with -う-u, -く-ku, -つ-tsu, or -る–ru. 
It becomes ja (the informal version of dewa) after the stem of te-form of verbs whose dictionary form ends with -ぶ-bu, -ぐ-gu, -む-mu, or -ぬ-nu.

After the stem of te-form of verb

そこへ行っちゃいけません。

Soko e itcha ikemasen.

You must not go there.

そんなことをされちゃ困ります。

Sonna koto o sarecha komarimasu.

I’ll be distressed if you do that.

-> See also ikenai, ja, naranai, tara