ja じゃ

Informal particle that is euphonic change of では dewa

1. After (adjectival) noun
(used for making the negative form of だ da/です desu)
Kore wa watashi no kaban ja arimasen.
This is not my bag.
(indicates a condition when followed by negative form of a potentila verb)
Kono tenki ja dekakerarenai.
We can't go out in this weater.
Kono konpyuutaa ja sore wa dekimasen.
It can't be done with this computer.

2. After the stem of te-form of verb whose dictionary form ends with: ぶ bu, ぐ gu, む mu or ぬ nu
Koko de asonja ikemasen.
You must not play here.

-> See also: ちゃ cha, では dewa, いけない ikenai,ては tewa