When is "私じゃ" used?


One of my Japanese friends wrote this sentence:

> 私 **じゃ** 歌えない

She meant that she can't sing a particular song. Is じゃ used in that sentence
the same way が would be used to put an emphasis on the subject, or is there a
different meaning to it?

Also, how informal is this form?




> 「[私]{わたし} **じゃ** [歌]{うた}えない。」

＝

> 「私 **では** 歌えない。」

「じゃ」 is the colloquial contraction of 「では」. More importantly, this 「では/じゃ」
placed directly after the subject of a sentence is quite _**nuanced**_.
Therefore, the plain and neutral 「は」 cannot replace it for the nuance.

It means that the subject (「私」 in this case) is _**not good enough to do
something for some reason**_.

**" I could not sing (this song), (but some other people probably could)"**

**" I am not good enough to sing (this particular song)."**

[![enter image description
here](https://i.stack.imgur.com/N6xLC.jpg)](https://i.stack.imgur.com/N6xLC.jpg)  
(source: [smilevideo.jp](http://tn-skr1.smilevideo.jp/smile?i=24855504.L))

「[僕]{ぼく}じゃダメですか？」 = "Am I not good enough for you?"




> 「[私]{わたし} **じゃ** [歌]{うた}えない。」

＝

> 「私 **では** 歌えない。」

「じゃ」 is the colloquial contraction of 「では」. More importantly, this 「では/じゃ」
placed directly after the subject of a sentence is quite _**nuanced**_.
Therefore, the plain and neutral 「は」 cannot replace it for the nuance.

It means that the subject (「私」 in this case) is _**not good enough to do
something for some reason**_.

**" I could not sing (this song), (but some other people probably could)"**

**" I am not good enough to sing (this particular song)."**

[![enter image description
here](https://i.stack.imgur.com/N6xLC.jpg)](https://i.stack.imgur.com/N6xLC.jpg)  
(source: [smilevideo.jp](http://tn-skr1.smilevideo.jp/smile?i=24855504.L))

「[僕]{ぼく}じゃダメですか？」 = "Am I not good enough for you?"

