Can Black capture en passant as well?

Yes, the rule is symmetrical. Black needs a pawn on the fourth rank, which is the fifth counted from Black's own side, and White has to push a pawn two squares from the second rank so that it lands beside it. A black pawn on e4 meets d2-d4 with exd3, landing on d3 and removing the pawn from d4.

What happens if I miss my chance to capture en passant?

It is gone. The right lasts for one move only, so if you answer the two-square push with anything else, that pawn can never be taken that way again. This is the only rule in chess that expires after a single move, which is why the en passant square is written into a FEN and then cleared on the next half-move.

Is en passant a real rule or something the app invented?

It is a real rule, written into the pawn article of the FIDE Laws of Chess, Article 3.7, and it has been part of standard play for centuries. Every arbiter, every book and every serious engine treats it as ordinary. The reason it feels invented is that most beginners meet it for the first time as the victim.

What is en passant in chess?

Black has just pushed the d-pawn two squares, d7 to d5. It is now sitting shoulder to shoulder with White’s pawn on e5, safely past the diagonal that pawn covers, and under the ordinary rules of capture there is nothing White can do about it. En passant says otherwise.

Black has just played d7-d5. White answers exd6: the pawn goes to the square Black skipped over, and the pawn on d5 is taken.

White’s pawn lands on d6 and the black pawn on d5 leaves the board, even though nothing ever set foot on d5. That is the whole of the move. You capture the pawn on the square it jumped across, not on the square it stopped on. En passant is French for in passing, and for once the name describes the thing exactly.

After exd6. White has a pawn on d6, the black d-pawn has gone, and Black can recapture with cxd6 or exd6.

The window is one move wide

This is the part that costs people material. The right to take en passant lasts for exactly one move. Black plays d7-d5; if White replies Nf3 instead of exd6, the pawn on d5 is safe for the rest of the game, and there is no coming back for it on move 20. Nothing else in chess expires like that, which is why a position has to record it: the fourth field of a FEN is the en passant square, and it reads d6 for one half-move before it turns back into a dash.

I found this out at nine, in the last round of a school tournament, when an opponent took my d-pawn with a pawn that had clearly gone past it. I called the arbiter over to complain. He put the position back, showed me the rule in the little booklet he kept in his jacket, and I lost the game and the argument in about ninety seconds.

What has to be true

Four things, all at once:

Your pawn is on its fifth rank. Rank 5 for White, rank 4 for Black, counted from your own side both times.
An enemy pawn moves two squares in one move, from its starting rank.
It lands directly beside your pawn, on the file next door.
You capture on your very next move.

Miss any one of them and the capture does not exist. The full conditions are worth reading once slowly, because the second one catches everybody: a pawn that reached d5 in two steps, d7-d6 and then d6-d5, cannot be taken this way. There was a moment when it stood on d6 and White chose not to take it there.

The same picture, reached by d7-d6 and then d6-d5. No two-square jump, so no en passant, and the pawn on d5 is simply safe.

Why the rule exists at all has a decent historical answer, and knowing it makes the rule stop feeling arbitrary.

Where it actually costs you a pawn

Two ways, and both of them happen under a minute on the clock.

The first is pushing two squares to run past something. Black has a pawn on e4, you have a pawn on d2, and d2-d4 looks like it gains space and slams the door. Then comes …exd3, the pawn is taken exactly as if you had played d2-d3, and you are a pawn down with your structure opened. In that kind of position d2-d3 is the honest move.

The second is not seeing your own chance. When an enemy pawn appears beside one of yours on the fifth rank, spend two seconds on it before you play anything else, because two seconds is all the time the rule gives you. A quick visual test: the two pawns have to be standing side by side on the same rank at the moment you look at them. A gap, or a difference of one rank, and there is nothing to think about.

It shows up in real openings more often than beginners expect. Take the Advance French, 1.e4 e6 2.d4 d5 3.e5: White’s pawn on e5 sits there staring at f6 for the next thirty moves, and when Black eventually breaks with …f7-f5, exf6 is on the table. Strong players usually decline it, because that pawn on e5 is the whole of White’s space advantage and taking hands Black the centre back. But you cannot decline a move you never saw.

And because the capture empties two squares (the one your pawn left and the one the enemy pawn stood on) while filling a third, it can open a line and give check by discovery. En passant checkmates exist. They are rare, and they are lovely when they land.

In the notation

You write it exd6, the same shape as any other pawn capture: the file your pawn came from, an x, the square it landed on. Older books tack e.p. on the end, so you will see exd6 e.p. in print, and PGN files never do. There is a page on what e.p. means if you have hit it in an old tournament book.

One more piece of practical advice, learned the hard way. If an opponent disputes the capture, do not argue about it over the board. Stop the clock, put your hand up, and let the arbiter read Article 3.7 out loud. It takes twenty seconds, you keep your position, and you are not the one who lost their temper.

If you want the move to stop catching you out, run through a set of en passant puzzles. Twenty positions where the capture is the whole point is usually enough to wire it in for good.