Oh xD Ok, thanks for your help :)

yes,I often use janai in conversation with friend. In my case, I use it positive meaning with my friend. But if I use janai to my boss,he will be angry.

It's not formal but you can use it in a "normal" conversation?

Janai is an interesting word in Japanese. It's not formal, but we often use Janai x 1000 in conversation. It contains positive & negative meaning. Basic meaning is "is not"! Sometimes we use janai to make sure about some staffs. これは、私の本じゃない。= This "is not" my book. これは、私の本じゃない? = "Isn't" this book mine? ケーキが焼けたね。いい感じじゃない?!= I've just made a cake! It "smells(/seems/sounds)" kinda good! あなたの子供を送っていったのはKeikoじゃない?! = "I think(guess)" Keiko picked up your kids in car... ...etc

ほんとにありがとございます　:)