hello!

in this diagram:

why black did that? what he profit from that?

and for the main question: what is the best answer for white? Bd2? Nc3? Nd2? and why?

this kind of position occur so many times in a games - there are rules of thumb for such a thing?

thank u,

I'm not so sure about the diagrammed position, but the usual reason for Bb4+ is that Nc3/d2 puts the Knight in a pin and after Bd2 black can either exchange pieces or play something like Be7. The main reason Black doesn't just play Be7 at once is that on d2 the Bishop is possibly misplaced - it can't develop to the b2 square and is blocking the Queen's view of the d4 pawn. In the position given though, Nc3 might be a fine move - since Bxc3 improves White's pawn structure.