"ja nai" after a verb


1) Is the plain "ja nai" after a verb grammatically correct?

[code]

       aru ja nai?
    
       wakatta ja nai?
    
[/code]

2) Why use ja nai, why not a negative form of a verb :

[code]

       aru ja nai? --> nai?
    
       wakatta ja nai? --> wakaranakatta?
    
[/code]

What's the difference?




あるじゃない{HLLLL} （ない{HL}）means "Look / Wow, there's (something)!".

あるじゃない{HLLLLH}？ means "By the way / You know, there's (something), right?".

あるじゃない{HLLLHL} means "It's not ある! / Shut up! Don't say ある!".

あるじゃないぃ{HLLLHLH}？ means "What? You mean, it's not ある?".

（だから）あるんじゃない{HLLLLL}（ない{HL}）means "(that's why) there's (something), no?".

あるんじゃない{HLLLLLH}？ means "I guess there's one".

あるんじゃない{HLLLLHL} means "It's not that there's one (but ...)".

あるんじゃないぃ{HLLLLHLH}？ means "(I thought there was but) you mean, it's not the
case?"

