Technically, both sentences are wrong! This is because you haven't inserted apostrophes which are obligatory here, not optional. Also, the second sentence starts with a small "i" and neither of them has a full stop. Sorry to be picky, but English does have rules! Now, assuming both are written correctly, as follows: I can't see you for a long time. or I haven't seen you for a long time. ...well, both are correct! Which one you want, however, depends entirely on the context. So, what kind of situation do you have in mind when you use these sentences? EDIT: according to your additional information, you are writing a letter to your friend and you are telling her that you will not be able to see her until 2 weeks have passed. Therefore, the FIRST sentence is the correct one to use: "I can't see you for a long time". You could also say "I won't be able to see you for a long time". By the way, I would also say that in English we do not generally consider 2 weeks to be a long time, so you might want to use a different expression here (although if you are used to seeing your friend every day then I suppose it might be okay to use "a long time". Again, it all depends on context). The second sentence "I haven't seen you for a long time" refers only to past events, and is therefore not suitable in your intended context.

-I haven't seen you for a long time. - I haven't seen you for ages

The second sentence...

第二个