Here is a study in caution. White just played the pawn capture d5xe6. Black must respond; but how? The important thing to notice is that White’s c and e pawns have moved, creating open diagonals for his queen. If Black’s f-pawn moves (to play f7xe6), the White queen suddenly will be able to check the Black king by moving to h5; if Black plays Bxe6, White’s queen can check with Qa4. The reason this matters, of course, is that Black has left a piece loose on a5. So Black mustn’t play f7xe6—as he did here, losing the knight to the queen fork Qh5+. He is better off using his d7 bishop to take the pawn on e6; for then if White tries a queen fork on the other side—Qa4+—Black can both move his knight to safety and block the check with Nc6. This is a common way that a good-looking queen fork can be spoiled: the forked piece moves to block the check, and suddenly both threats are gone.
Another way to see this, naturally, would be to start by keeping tabs on Black’s loose pieces. Here he has a loose knight on the fifth rank. Loose pieces on the middle ranks often are vulnerable to double attacks of this type by the queen, especially early in the game; in view of this vulnerability, Black should think carefully before opening any fresh lines to his own king. White can treat the situation as an opportunity; in effect Black's f7 is subtly pinned in place, since if it steps forward White has a queen fork.
Incidentally, f7xe6 also would be a weak move for Black on strategic grounds. It weakens the pawn cover on his kingside, where he might want to castle; and it creates a little pawn island—an “isolated” pawn with no fellow pawns on the files to either side of it—that will have to be protected by a Black piece from now on.