Can't figure out this move...

Just learning the basics, so go easy on me . Finished a game and was told afterword that in this position, 18. Nxc7 (capturing a pawn), was the best move to make. I don't see how this could be, since blacks queen can just recapture, and (seemingly) no harm is done to black.

The computers line from here apparently goes: 18...Nxd4  19. Nxa8 Bxa8  20. Qxd4  f5  21. e5

I can't see why blacks queen wouldn't just take the knight. Can anyone show me what I'm missing?

It should be there, completed game against the computer.

Qxc7 Ne6+ wins the black queen. 

No, bot games don't show up in your profile. As for the "perfect" move, I guess a reason for why the computer might consider this a perfect move would be because from there the knight can pull Nxa8 (capture Black's rook.) But that doesn't make any sense since the knight would just get captured by black's queen before anything else could happen.

Thank you, that's been bothering me!

It's not the easiest thing to see for a novice player, since it's not easy for new novice players to observe the new position of the queen after the capture Nxc7.

In any case, if the queen does not capture the knight, that same knight can head to e6 so that one knight goes to e6 anyway - and it's going to be quite a big issue for Black - and that's why the position is rated as +5.33.

Most importantly, when you are on the engine analysis page, you can make the move(s) that you don't understand and see how the engine responds. After Nxc7 simply play Qxc7 for black.