Why i couldn't do the En passant move ?

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escapement

I was playing black and after my opponent played pawn to from g2 to g4 why i couldn't do the en passant move to capture his pawn with my pawn on h3. 
Any explanation with be appreciated . thanks for your time.

urk
Your pawn has to be on your 5th rank capturing a pawn that moved two squares.
Your pawn was on your 6th rank.
Lbjon
Your pawn needed to be on h4
oregonpatzer

What Lbjon said, your pawn needed to be on h4 to do this.  You can't do it with the pawn on h3.