A well motivated exchange. Black would have been better to have taken the knight, probably. A piece ahead for just one pawn, White should welcome exchanges, and this is a good one which will cause some disruption to Black's Q-side after 13...exd5 14.Nc3.

Attacking the loose d-pawn. To defend it Black will have to interfere with his knight's development.

... so he doesn't defend it. But he really should.

Odd. I don't see any purpose to this move. Maybe Black was hoping for 16.dxc5 Bxa1 - winning a rook. But White could have retained a large winning advantage by continuing 17.Bg5 f6 18.Bh4 Bb2 (say, to escape capture) 19.Qb3! ... and the bishop falls after all.

(?!) A very interesting move. Best was the simple 16.bxc5, but, although White is now about to lose a piece (16.Bg5 Bxg5), there are compensations: after 17.dxc5 or 17.Nxg5 Qxg5 18.dxc5 ... White is a pawn ahead with a fine game. Having said that, Black SHOULD take the Bishop!

A little bit TOO tricky. Simply 17.Nxf6ch or 17.Bxf6 were both far better options.

White ought to have exchanged on f6 first, or played 18.Re1 and THEN exchanged on f6. Now Black will be material ahead, with a big advantage.

Careless.

A rook and a pawn down - not looking encouraging, after such a great start.