Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...a6/4. Ba4/4...d6/5. d4

5. d4?! is premature.

After 5....b5!, Black unpins their knight and brings another attacker to d4 with tempo. 6. Bb3 Nxd4 and White has lost an important centre pawn. (If 6. d5? to counterattack the knight, 6...bxa4 7. dxc6 Ne7 and White has only opened up the b-file for Black's rook. Black will win the pawn on c6 shortly.)

If White thinks they can win back the pawn with 7. Nxd4 exd4 8. Qxd4?? then they have walked into the classic Noah's Ark trap. White's bishop on b3 is low on squares. If Black could catapult their pawn to c4, they would trap it, and White's queen on d4 now gives Black a way to get their pawn to c4 with tempo. The full line, including White's death throes, is 8...c5 9. Qd5 (threatens Qxf7#) Be6 (if 9...c4?? 10. Qxa8) 10. Qc6+ Bd7 11. Qd5 c4-+.

The blunder only comes with 8. Qxd4??. There are a few alternative eighth moves that avoid the trap. When chess master's go into this line it is with 8. c3! in mind: the idea is after 8...dxc3 9. Qd5, White can force a draw by repetition between Qc6+ and Qd5 threatening Qxf7#. If Black is unwilling to allow the draw, they have to accept an inferior position.

An imaginative alternative is 8. Bd5!?, threatening Black's rook, and 8...Rb8 9. Bc6+ Bd7 10. Bxd7 Qxd7 11. Qxd5 to recover the pawn.

5...Bd7?! to end the pin that way lets White choose to transpose back to the main line of the defence with 6. c3, or seize more space with 6. d5 and c4.

Wikipedia has related information at Ruy Lopez#Modern Steinitz Defence.