How is this not mate?

How is this not mate? Time just kept on ticking and I ultimately won by resignation. 

en passant is a thing buddy

Haha, so he captures with f4! Neither him or me found it.

Your opponent can play en passant here

When you advance a pawn two spaces forward on its first move of the game and it lands on a square right next to your opponent's pawn, your opponent can actually capture that pawn by sliding the pawn directly underneath the pawn, capturing it as if it had only moved up one square.

Example:

So your move is not checkmate because your opponent can play fxg3 or hxg3, capturing the pawn en passant. Hope that helps 🙃

Or hxg3. He could’ve done en passant either way.

Hola

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I learnt the en passant the hard way after two encounters with it, one of which appeared in very similar circumstances in an online game on another site (but it was a rook guarding instead of a queen), with the "checkmate" coming on the queenside.

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My opponent also did not know it, and (I believe after looking up chess rules) finally found it close to 10 minutes later. I was like, "HEY! THAT IS AN ILLEGAL MOVE! HOW CAN THE SITE ALLOW SUCH ILLEGAL MOVES?".

Because of enpassant