Queen's Gambit Declined
Black's best continuation is 8...Bb4, pinning the c3-knight. 8...Bd6? ignores e4 altogether, and doesn't prevent 9.Ne5 (9...Bxe5? 10.dxe5! Ne4 11.Nxd5!!). White's best continuation is either 9.e4! dxe4 10.Nxe4 or 9.0-0 Bc6 10.Ne5 0-0 11.f4! Once White castles, 9...Bc6 is necessary to retain e4. After 9...0-0? White is free to advance: 10.e4! dxe4 11.Nxe4 Nxe4 12.Bxe4. Bxe4 is uncomfortable for Black, and it is better to avoid 11...Nxe4. However, White is also threatening 12.Nxf6+ Qxf6? 13.Bg5! Qe6 14.Re1! Therefore, 11...h6 is better than 11...Nxe4.
12...Bc6 challenges Be4, but is countered by 13.d5! After 13...Bb7?, the wasted tempo of 6...Bd7? is apparent. Meanwhile, Black actually needs 13...Bd7, as White is planning 14.Bxh7+. The best continuation is 14...Kxh7 15.Ng5+ Kg6 16.h4 Qd7, which can be compared with 13...Bd7 14.Bxh7+ Kxh7 15.Ng5+ Kg8 16.Qh5 Bf5!
14...Kxh7 loses, but even worse is 14...Kh8? 15.Ng5 Qf6?? (better is 15...Nd7|16...Nf6) 16.Qh5 Qh6 17.Nxf7+
