I would like to know what's the difference of じゃん and だろう/でしょう.

the meaning I know it's similar, but when I should use each of them?

Is it same to say:

これはすごいじゃん! = これはすごいでしょう! = これはすごいだろう!

昨日、楽しかったじゃん? = 昨日、楽しかったでしょう? = 昨日、楽しかっただろう?

できるじゃん! = できるでしょう!



じゃん is じゃないか, i.e., ではありませんか

When you say "すごいじゃん", you actually affirm "that's great, isn't it?", you give your opinion, and don't care about any one else. The intonation is that of a strong affirmation.

だろう/でしょう is rather an introspective form, or an "open to suggestions" affirmation. When you say "すごいでしょう", you say "that's great, isn't it" and you look at the people around you, expecting an agreement, a reaction. The intonation is that of a question.

できるじゃん! is something you say while doing it: "see, I can do it!". Though more naturally, I'd say "できたじゃん" or "できているじゃん".
できるでしょう is something you say before he does it: "I'm pretty sure he can do it, can't he?"
(If I were to say "I'm pretty sure I can do it", I'd say it differently, that's why the subject changes in my example.)

Also, grammar dictates that you can't say すごいだろう, so usage has it like "すごいんだろう", when you want to leave politeness apart (even though でしょう as in the example above is perfectly ok in informal conversations).

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    Just a question on できたじゃん, is that the shortened version of できたんじゃない with んです or is it perfectly fine grammar meaning something else? Was always under the impression that it needed a 「ん」 in there – Mark Hosang Aug 6 '11 at 10:33
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    @Mark: できたじゃん = できたじゃない ≠ できたんじゃない. For the difference between できたじゃない and できたんじゃない, see 「~たじゃない」 expression in spoken Japanese. – Tsuyoshi Ito Aug 6 '11 at 15:33
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    the grammar would also allowed me to say 楽しかっただろう ? Maybe the correct form would be 楽しかったんだろう。In spoken conversation, there's much problem with these things? (i mean, japanese also commit this mistakes?) – daniel tomio Aug 6 '11 at 21:57
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    @Axioplase Btw why is "すごいだろう" not grammatical? [adj-i][だろう] is grammatical isn't it? – Pacerier Mar 30 '12 at 0:43
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    @Axioplase, could you please tell me (or point me to a resource) why it would be 楽しかったろう? I would expect it to be as daniel tomio wrote it. That said, I'm not good at Japanese yet. – batbrat Aug 29 '17 at 16:39

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