Qoko wrote:
@kthprog As thumb rule, if you aren't sure why just check the analysis itself! It gives you the better move, and indeed, chess improvements consist of finding a stronger winning move or not losing so that accounts for both (ie 9.0-0? Bxc3 -+ and 15....Qxe5? +- (also thumb rule, the exposed queen on e5 in the French structures is rarely good)).
As for the opening, anti-positional moves are played; I've played the French for quite a while and the structure turns out non-French early on.
How is the French defense generally supposed to be played and why? Thanks 
If you cannot interpret why the engine says your move is a blunder or a mistake then the best way to understand it is by playing a few moves according to the variation which the engine suggests.