Carlsen vs. Nepo this morning, why didn't Nepo win the pawn? On move 19 Carlsen played Nd5. Why didn't Nepo win the pawn with Nxd5? If Bx, then Qxb4, or if px, then Qxb4 attacks the Bc4 still and Bxa6 then Nc5 and move the rook.

 [FEN ""]
 [Event "Tata Steel Rapid &amp; Blitz"]
 [Site "Kolkata"]
 [Date "2019.11.22"]
 [Round "2.3"]
 [White "Carlsen, Magnus"]
 [Black "Nepomniachtchi, Ian"]
 [Result "1-0"]
 [ECO "B51"]
 [WhiteElo "2870"]
 [BlackElo "2773"]
 [PlyCount "87"]
 [EventDate "2019.??.??"]
 [EventType "blitz"]
 [WhiteTeam "Norway"]
 [BlackTeam "Russia"]
 [WhiteTeamCountry "NOR"]
 [BlackTeamCountry "RUS"]

  1. e4 c5 2. Nf3 d6 3. Bb5+ Nd7 4. d4 cxd4 5. Qxd4 a6 6. Be2 Ngf6 7. O-O e5 8. Qe3 Nc5 9. Nfd2 b5 10. c4 bxc4 11. Nc3 Bb7 12. Bxc4 Be7 13. b4 13... Ne6 14. Nb3 Rc8 15. Na5 Qd7 16. Qd3 O-O 17. Be3 Bd8 18. Nxb7 Qxb7 19. Nd5 Nc7 20. Nxf6+ Bxf6 21. Rab1 Rfd8 22. Rfd1 h6 23. f3 Qc6 24. Qb3 Qe8 25. a4 Rb8 26. b5 axb5 27. axb5 Ne6 28. b6 Bg5 29. Bf2 Nd4 30. Bxd4 exd4 31. Rxd4 Rdc8 32. b7 Rc5 33. Bd5 Qe7 34. Rc4 Qc7 35. Rxc5 dxc5 36. g3 Bf6 37. Kg2 Qe7 38. Qb6 Be5 39. Qc6 Kh7 40. f4 Bd4 41. Qc8 Qd6 42. Qf5+ Kh8 43. Kh3 Qa6 44. e5 1-0
The solution to that is much easier than it looks, and a fairly common type of position in practical play. After 19.Nd5! taking the nice outpost square, if black responds with 19…Nd5 then white can simply play 20.Bd5 Qb4, then Qxa6 just leaves black positionally lost despite even material. In positions like that where you have a rook behind the a-pawn, and the bishops both pointing at the queenside, the pawn on a2 is going to run fast and hard, and soon cost black heavy material.

  [FEN ""]

  1. e4 c5 2. Nf3 d6 3. Bb5+ Nd7 4. d4 cxd4 5. Qxd4 a6 6. Be2 Ngf6 7. O-O e5 8. Qe3 Nc5 9. Nfd2 b5 10. c4 bxc4 11. Nc3 Bb7 12. Bxc4 Be7 13. b4 13... Ne6 14. Nb3 Rc8 15. Na5 Qd7 16. Qd3 O-O 17. Be3 Bd8 18. Nxb7 Qxb7 19. Nd5 Nxd5? 20. Bxd5 Qxb4 21. Qxa6 {And from here, it is easy to see what kind of troubles black has.}