pawn took sidewards?

https://www.chess.com/learn-how-to-play-chess

Playing for fun is perfectly fine... now that you've learned en passant, it might even be more fun.

The good news is that's usually the last rule people learn, so there are probably no more tricky new rules out there you don't know about

Another great trick is Castling!

What about the King Twirl?

Shh, we're not supposed to talk about that in front of new players.

He didn't say he was new... just that he hasn't yet turned pro.

Oh, right.

Well a lot of people say it's best to start with spinning tops, but I say there's no harm in jumping right in and twirling with a 4.5 inch tripple weighted majesty.

Has he encountered the pawn pirouette yet? 

Or the Rook Ricochet?

Personally, I found those the hardest. 

Or the Knights a leaping?

Apparently en passant has been a part of the game since the 15th century, the same time people started moving pawns two squares at time to speed up the opening. In light of that you could think of en passant as an obvious rule patch to make sure double pawn moves wouldn't let pawns get past enemy pawns on neighbouring files without threat of capture, which probably would have changed the game quite drastically (although they can if for instance).

Apparently game balancing as a concept is apparently not a modern phenomenon.

The in-game justification, if you will, is that moving a pawn two squares sort of counts as two moves in one, and capturing en passant is to capture the pawn in question after the first of the two.

I don't know any of those...are there tutorials on this stuff?

I have a feeling you might be in on it, but in case you're not: All those are jokes.

The basic moves of the various pieces, castling, double pawn moves, en passant, checks, checkmate and the different criterias for draws make up all the rules there are in chess.

If you won't take my word for it (and you kind of shouldn't, I'm just some rando on the internet, after all), just look up the rules of the game somewhere and see for yourself.

https://www.chess.com/article/view/his-pawn-cheated-and-killed-my-pawn

R.I.P..  the Unknown Pawn

https://www.chess.com/article/view/his-pawn-cheated-and-killed-my-pawn

EDIT: Did not read page 2 yet at my time of posting; had not realised that @1_a31-0 has posted this.

I learned this one the hard way after wondering how the position was not checkmate, even though it clearly looked like a checkmate - the king can't run, and the pawn can't be captured.

Or, is it?

Coincidentally my opponent had not been aware of en passant as well.