How is 〜んじゃない different from 〜じゃない?


Is it possible to say ～んじゃない？ If it is, how it differs from ～じゃない？

In what situation I should use ん/の for it, and what does ん/の express?

Examples:

> **[~い + ん]**  
>  いい **ん** じゃない  
>  行きたい **ん** じゃない  
>  吸えない **ん** じゃない
>
> **[~な + ん]**  
>  好き **なん** じゃない  
>  先生 **なん** じゃない
>
> **[verb(~る/~た) + ん]**  
>  する **ん** じゃない  
>  食べなかった **ん** じゃない

Is it all possible?  
In my opinion, the んform expresses there is something implicit or implied.
like a desire, or a sad comment, etc ; but saying it in a soft way.

Also:

  * じゃん is the shorten form of じゃない.  
Can I say ～んじゃん, then?  

> 楽しいんじゃん  
>  便利なんじゃん  
>  するんじゃん
>




Yes, you can say 〜んじゃない. The ん/の performs its usual function indicating that
you are explaining some fact ([see my answer to this previous
question](https://japanese.stackexchange.com/questions/506/use-of-in-
questions-not-seeking-a-yes-no-answer/521#521)). 〜じゃない here turns the sentence
into a tag question ([see Derek's answer to your previous
question](https://japanese.stackexchange.com/questions/1481/usage-
of/1485#1485)).

> A: (I've already gotten five great answers to questions I asked on the
> Japanese.SE site!)  
>  B: ええ、便利なんじゃない？

Here B uses ん to relate 便利 to A's statement, thus explaining why A is so
happy. "Yes, it's convenient, isn't it [and that's why you are excited about
it]!"

> A: (I'm so hungry...)  
>  B: え？ さっき食べたんじゃない？

Here B uses ん to explain B's own surprise at A's statement. "What? Didn't you
just eat [and this is why I'm surprised that you're hungry so soon]?"

As for 〜んじゃん, yes, you can use it. But 〜じゃん is pretty blunt/masculine, so
don't use it with your boss.




Yes, you can say 〜んじゃない. The ん/の performs its usual function indicating that
you are explaining some fact ([see my answer to this previous
question](https://japanese.stackexchange.com/questions/506/use-of-in-
questions-not-seeking-a-yes-no-answer/521#521)). 〜じゃない here turns the sentence
into a tag question ([see Derek's answer to your previous
question](https://japanese.stackexchange.com/questions/1481/usage-
of/1485#1485)).

> A: (I've already gotten five great answers to questions I asked on the
> Japanese.SE site!)  
>  B: ええ、便利なんじゃない？

Here B uses ん to relate 便利 to A's statement, thus explaining why A is so
happy. "Yes, it's convenient, isn't it [and that's why you are excited about
it]!"

> A: (I'm so hungry...)  
>  B: え？ さっき食べたんじゃない？

Here B uses ん to explain B's own surprise at A's statement. "What? Didn't you
just eat [and this is why I'm surprised that you're hungry so soon]?"

As for 〜んじゃん, yes, you can use it. But 〜じゃん is pretty blunt/masculine, so
don't use it with your boss.

