Hello. I'm an English beginner. I'd like to ask one question.

"I'll see you two weeks later", does it sound unnatural to you? If so, please tell me the reason. I'm wondering why it is wrong, because I know you often say "See you later" or "I'll see you later". I know "I'll see you in two weeks" is the correct form. What I want to know is just why "I'll see you two weeks later" is wrong.

Thank you in advance and I wish you all a Merry Christmas!
I think we mainly use 'later' when we are not being very precise about the time/date. Two weeks is a precise time.
"I'll see you two weeks later"
isn't idiomatic (not used).

Just do a search at Google with:

"I'll see you in two weeks"
785 hits
"I'll see you two weeks later"
2 hits

Such a large disparity should tell you you're in the wrongEmotion: sad

Another option:
"I'll see you in two weeks time"

I'll see you in two weeks!

http://sportsillustrated.cnn.com/inside_game/grant_wahl/news/2003/05/31/wahl_soccer /
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The word 'later' is relative when used in this context... (ie... 2 weeks later than what or later than when? Not sure this helps, but hopefully, it will...
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