Chess Opening Theory/1. e4/1...Nf6/2. e5/2...Nd5/3. d4/3...d6/4. Nf3/4...Bg4

4...Bg4 is the main line: this pins the knight so prevents it from recapturing after ...dxe5.

If 5. h3, the Panov variation, then Black is happy to trade the bishop for the knight. 5...Bxf3 6. Qxf3 dxe5 (simultaneously defending the knight on d5) 7. dxe5 e6 ⩲, and while material is equal, Black has weakened White's centre.

5. Be2 is the main line. This unpin the knight to resume control of e5.

The idea of 5...Bxf3? fails in this line, because 6. Bxf3 dxe5? 7. c4 Nb6 8. Bxb7 winning the exchange +-. Taking the pawn first, 5...dxe5 6. Nxe5 Bxe2 7. Qxe2 ± gives White a slightly superior version of the Larsen variation (4...dxe5), and it's difficult for Black to make progress developing their position. White has ideas like Qb5+ to fork the b7 pawn (e.g. 7...e6 (to develop the bishop) 8. Qb5+).

In the main line Black leaves the tension and makes modest moves to develop their position in the available space, continuing 5...e6 6. O-O Be7 7. c4 Nb6 8. Nc3 O-O.

5. c4 now is the Alekhine variation of Alekhine's defence. The knight's only safe square is b6 (5...Nb4 still loses to Qa4+). After 5...Nb6, it is now inaccurate to interrupt the pin with 6. Be2?! because 6...dxe5 7. Nxe5 Bxe2 and White has to take back on e2 with their king, losing the right to castle, else drop the d-pawn: in this line, White made Black's knight move already, and so allowed Black's queen to see d4. Therefore White must continue 6. exd6, and either recapture may be played.

1.e4 Nf6 2.e5 Nd5 3.d4 d6

Wikipedia has related information at Alekhine's Defence.