Caruana vs Carlsen game 10 (WCC) why not 18...Nxb6?

Question

In game 10 of the Caruana-Carlsen WCC, why did Carlsen (black) not take the terribly annoying and strong dark squared bishop on b6 on move 18 when given the chance? Instead, Carlsen moved the queen with Qe8.

The position is shown in the following diagram:

 [title "Caruana vs Carlsen, game 10, why not 18...Nxb6 instead of 18...Qe8"]
 [fen "r1bq1rk1/1p1nb1pp/1B1p4/P2Pp3/2N2p2/8/2P1BPPP/R2Q1RK1 b - - 0 1"]

Answer 1

From a purely positional point of view, 18...Nxb6 would have been a terrible mistake, as it would have allowed white to then forcefully trade the light squared bishops with the immediate Bg4 move, leaving exposed all the light square weaknesses that black has created with the d6-e5-f4 setup.

These weaknesses (created holes) would then have been permanently exploited by white's knight (and queen), in other words, white's minor piece (b6 knight) would have dominated the game and been virtually untouchable on light squares for the rest of the game. Notice that next to the knight controlling key light squares from b6, it's also for example quite feasible for white to re-route the knight to e4, established centrally (permanently) and constantly eying black's weak d6 pawn.

And as for black, Carlsen would have simply been left with a bad dark squared bishop hemmed in by his own structure. Therefore, Carlsen chose wisely to play Qe8 instead of Nxb6, thus, preserving his c8 bishop as Bg4 no longer forces a trade thanks to the well placed knight on d7.

In summary, this is a purely positional decision where black is well aware that having weakened their light squares, any scenario with the knight vs dark square bishop would be unfavorable for them, and arguably, at that level even objectively lost! Instead having kept both the d7 knight and c8 bishop, black has in fact preserved two pieces that can provide light square control.

The following situations would have ensued had Carlsen played 18...Nxb6:

18...Nxb6 19.Nxb6 Ra7 20.Bg4

which would have led to positions of the following nature, infested with light square holes:

Answer 2

After 18 ... Qe8 the bishop on b6 is attacking thin air and isn't very annoying.
After 18 ... Nxb6 19. Nxb6 the knight on b6 is very annoying indeed. The a8 rook is forced to a7 and black's pieces are becoming uncoordinated.
After 18 ... Nxb6 19. axb6 Rxa1 20. Qxa1 black is in trouble. White's b6 pawn is looking dangerous, black's b7 pawn is going to come under attack and his position is cramped and uncoordinated.

Answer 3

Several reasons I can think of:

• black is obviously playing on the kingside and Qe8 is a useful move aiming to transfer the queen to f7, g6 or perhaps h5 later. Also it might be useful to push e4 later.
• the bishop on b6 is not annoying. At the moment it attacks the black queen but that's about it. To some extent it is also blocking other white pieces. Also the bishop does not have many squares and could (if necessary) be taken later.
• the position after 18...Nxb6 19 Nxb6 is worse, because now you have a knight on b6 and the pawn on b7 is more prone to become weak. Also the light squares are much weaker then.