In the following Ruy Lopez opening;
[FEN ""]
1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Ba4 Nf6 5.O-O Be7 (5... Nxe4)
Now the question, instead of Be7 why does Black not play Nxe4 as e4 is not defended?
Thanks
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Sign up to join this community5...Nxe4 leads to the Open Spanish. It's a respectable opening with some very long lines, as is typical of the Ruy Lopez.
One thing it does not do though is win a pawn, because White can easily win it back after 6. Re1 (the main line is actually 6. d4 because Black cannot hold on to the pawn anyway). The knight is threatened and will eventually be forced to move, after which the e5-pawn falls.
Here's an illustration of what might happen if Black tries to hold on to the pawn:
[FEN ""]
1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 Nf6 5. O-O Nxe4 6. d4 exd4 {Or the e5-pawn is lost.} 7. Re1 {Threatening the e4-knight. It is pinned to the king.} d5 {Or 7...f5, which is also met by 8. Nxd4 except this time it threatens Nxf5. 7...Qe7 would be even worse, since after 8. Nxd4 the knight is still pinned and White threatens to exploit it with 9. f3.} 8. Nxd4 {White threatens to take twice on c6 and also to exploit the pin with 9.f3. Black cannot keep the extra material.}
White does not lose a pawn here as e4 was indirectly defended via castling. For example, after ...Nxe4 6. Re1 Nf6 white can take the e5 pawn to restore material balance.
William: (1) Taking is the Open Spanish, which is perfectly playable for Black. (2) But, as others have noted, Black doesn't keep the pawn. (3) If Black tries to keep the pawn, he can get himself into trouble along the e-file. A relevant game to look at is Fischer-Trifunovic 1961. Another is Daskalov-Belchev 1975. Be sure to look at them with an engine, so as to spot the weak moves--especially in the latter game.
Edited to add: I have the game saved here: https://www.chess.com/a/kd2jsHzW1KMk . In case you cannot see it, here is the game score:
[Event ""]
[Site "Bulgaria"]
[Date "1975.??.??"]
[Round "?"]
[White "Daskalov Georgi (BUL)"]
[Black "Belchev B"]
[Result "1-0"]
[ECO "C80"]
[WhiteElo "2355"]
[BlackElo "0"]
[Annotator ""]
[Source ""]
[Remark ""]
[FEN ""]
1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 Nf6 5. O-O b5 6. Bb3 Nxe4 7. d4
exd4 8. Re1 d5 9. Nc3 dxc3 10. Bxd5 Bb7 11. Bg5 f6 12. Rxe4+ Be7 13.
Bxf6 gxf6 14. Ng5 fxg5
15. Qh5+ Kd7 16. Qh3+ Ke8 17. Rd1 Qd6 18. Re6 Qf4 19. Qxc3 Rf8 20. Bxc6+ Bxc6
21. Qxc6+ Kf7 22. Rxe7+ Kg8 23. f3 Rad8 24. Qe6+ Kh8 25. Rxd8 Rxd8 26. Qe5+ Qxe5
27. Rxe5 h6 28. Rc5 Rd1+ 29. Kf2 Ra1 30. a3 Ra2 31. b3 Rxa3 32. Kg3 Ra1 33. Kg4 Rg1 34. g3 Rg2 35. h4 gxh4 36. Kxh4 a5 37. Rxc7 a4 38.
bxa4 bxa4 39. c4 a3 40. Ra7 a2 41. f4 Rc2 42. Kh5 Rxc4 43. Kxh6 Rc6+
44. Kg5 Rc2 45. f5 Kg8 46. Kg6 Kf8
47. g4 Rf2 48. g5 Ke8 49. f6 {Game may have continued...} (49. f6 a1=Q 50. Rxa1 Kd7 51. f7 Ke6 52. Re1+ Kd5 {+M20}) 1-0
Alas, I am not sure where I first saw it--possibly in an article somewhere.