How do you use 'en passant'? I have been playing chess for years but I was just introduced en passant only recently. I keep on forgetting when to play this weird move. I keep on asking myself "is it on the 5th row or the 4th row" If there is an easy way to remember, please help.
Perhaps it helps if you remember the (presumed) reason for having this special move. 

Originally pawns were only allowed to make single steps. This meant that pawns on adjacent files could not pass each other without running into the danger of being captured.

When the double step was introduced, pawns on adjacent files could suddenly pass each other (which would have changed the evaluation of many positions most notably in endgames). They could pass each other exactly in one case: white pawn on 5th row black pawn making double step (or equivalently black pawn on 4th row white pawn making double step).

So bottom line if you see a pawn passing your pawn without your pawn ever having a chance to capture it, it is time for en passant.