Why does White play Qxd4?

Question

This puzzle on chesstempo caught my eye. We play as Black:

[FEN "r2qk2r/1p1bppbp/p1np2p/4P2n/Q8/2N2N1P/PPP2PPB/R3KB1R w - - 0 1"]

1. exd6

My solution is: if we play 1... Nb..., a discovered attack by the bishop on a White queen takes place. Therefore, we shall use this knight wisely. The interesting thing we may do is threatening a fork on the king and on the rook (2... Nc2+) playing Nbd4. It is the right move:

[FEN "r2qk2r/1p1bppbp/p1np2p/4P2n/Q8/2N2N1P/PPP2PPB/R3KB1R w - - 0 1"]

1. exd6 Nbd4

I wanted to make the White queen move so it would be no longer defending the c2, making the fork possible (that would something like 2. Qc4):

[FEN "r2qk2r/1p1bppbp/p1np2p/4P2n/Q8/2N2N1P/PPP2PPB/R3KB1R w - - 0 1"]

1. exd6 Nbd4
2. Qc4 Nc2+
3. Kd2 Na1

And yet, the White plays 2. Qxd4 and gives Black the queen. Then White captures the bishop:

[FEN "r2qk2r/1p1bppbp/p1np2p/4P2n/Q8/2N2N1P/PPP2PPB/R3KB1R w - - 0 1"]

1. exd6 Nbd4
2. Qxd4 Bxd4
3. Nxd4

The only reason as to why White would answer this way I found afterwards is White not losing a piece (they get to capture the knight, which couldn't have been captured after the fork) and losing the same material (-Queen + Bishop = -8 + 3 = -5 = -Rook). Yet this seems a bit weak. If I were White, I would have played the other way. Yet, I am no old hand in chess, so there is no good reason to trust my gut. What is more important here: not to lose the queen, or not to lose the pieces? Anyway, why does White play 2. Qxd4?

Answer 1

1. exd6 is bad because it opens up the bishop on g7. This would happen anyways, but it just made Black's job easier. It also completely opens the centre, which is not a good thing to do, especially when the king hasn't castled. Finally, it ignored the bishop that is indirectly threatening the Queen.

The correct response is 1... Nd4 which enables the Bishop to (directly threaten the queen) and places the knight in a central square, threatening at the same time the c2 pawn.

Now white has two options:

1. Qc4, which is a move that beginners would prefer as they cannot live without their queen. Black would reply with Nc2+ and after some moves here's how the position would look like:

   [Variant "From Position"]
   [FEN "r2qk2r/1p1bppbp/p1np2p1/4P2n/Q7/2N2N1P/PPP2PPB/R3KB1R w KQkq - 0 1"]
   
   1. exd6 Nd4 2. Qc4 Nxc2+ 3. Kd1 Nxa1
   

   Now White's position has multiple disadvantages:

   • The centre is open and the white king can't castle!
   • Material difference
   • The rook and the bishops aren't fully activated compared to the respective Black pieces.

2. Qxd4, a move that more experienced players would prefer. Here's how the position would look like after Nc2+, etc:

   [Variant "From Position"]
   [FEN "r2qk2r/1p1bppbp/p1np2p1/4P2n/Q7/2N2N1P/PPP2PPB/R3KB1R w KQkq - 0 1"]
   
   1. exd6 Nd4 2. Qxd4 Bxd4 3. Nxd4 O-O 4. O-O-O
   

   Now White is much better as:

   • They have castled.
   • Knights and the black-coloured bishop are active.
   • The white-coloured bishop will be developed probably in the next move, connecting the rooks at the same time.
   • They can attack the Black king (opposite castles)!
   • The dangerous black bishop on g7 is gone!
   • Bishop on d7 is inactive.

I believe you should try the move that will make your play easy later in the game and not Qxd4 just because the engine says so.