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 ...
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 ...