I struggled a lot on this:
lichess.org/training/61678after bishop x g8, rook x g8, queen b3,
black plays 0-0-0, which is a mistake, as stockfish confirms in the subsequent analysis
Well, It doesn't really matter at that point. Black is losing a piece either way.
0-0-0 is definitely not black's best try, but it doesn't change the objective outcome of the position.
d5 instead of 0-0-0 is definitely a better try, but black is still losing. It's just not as straightforward as taking a piece in one move :)
@a_pleasant_illusion It was exactly thinking about d5 that put me in difficulties. How would you continue, as white, after ...d5?
I understand white is in anycase in a better position, but I do not see an immediate advantage
Thanks!
So, after d5, white plays a3.
Only squares for the knight are a6 and c6.
If Na6 there's no good response to Qxb7, hitting the a8 rook and a6 knight.
After Nc6, white has Nc3!.
This threatens Nxd5, which does all the following:
1) Wins a pawn
2) Threatens the e7 queen
3) Threatens Nxc7+ (it's supported by the Bh2, even, so defending it once isn't enough), which in turn A) wins another pawn, B) forks a8 rook and e8 king, AND C) discovers an attack from the b3 queen on the g8 rook.
As it turns out Black just has no response to this that doesn't result in a losing position.
So, to set it up again, we have 11.Bxg8 Rxg8 12.Qb3 d5 13.a3 Nc6 14.Nc3
If black tries 14...Be6 (so Nxd5 can be met by just trading it off), the c6 knight is now only defended by the b7 pawn, so 15.Qxb7 just wins material because of the fork of the c6 knight and a8 rook (with the bishop on d7 this wouldn't work the same way because the knight would still be protected by the bishop).
On that note, it should be pointed out that 14.Qxb7?? instead of 14.Nc3! would actually lose because of 14...Rb8 (c6 knight is still protected), and now if 15.Qa6 there's 15...Nxd4!, and if 15.Qxc7 there's 15...Rc8 16.Qb7 Nxd4! (I won't go into those tactics in too much detail; you can certainly work them out if you're interested though).
Back to the position after 11.Bxg8 Rxg8 12.Qb3 d5 13.a3 Nc6 14.Nc3:
If black instead of 14...Be6 tries to put everything on defense of c7 with 14...Rc8 15.Nxd5 Qd8, then white can use the discovery on the g8 rook mentioned earlier by playing 16.Nxc7+, forcing 16...Ke7, since the g8 rook is hanging.
At that point white would be winning handily, being up a pawn with a crushing attack that will force more material gain shortly, either with Ng5-e6 ideas, or Qd5 with Bd6+ ideas. Even Qxb7 or 0-0 would also win there.
Anything else on move 14 for black just allows Nxd5 followed by Nxc7+ winning decisive material very simply.
Hopefully that helps!
@a_pleasant_illusion thanks for your excellent analysis.
It's unclear why in the puzzles (or, at least, in this specific one), the machine plays a move (in this case, 0-0-0), that Stockfish does not even take it into account in the subsequent analysis.
Kind regards
F.