Hello, I've been trying to learn about cohomology theory (particularly in relation to Lie algebras, the Hochschild cohomology and deformations of algebras) and I never seem to get very far. It seems there's a bit of a jump between knowing what Ext and Tor mean and actually feeling comfortable with them and feeling confident that I can actually calculate them, and knowing when you can calculate them. Maybe I'm just being lazy. I have to keep looking up the definitions of each every time I see them being used in something (I'm thinking of making myself a little card to put in my wallet). Does anyone have some tips? Are there some exercises that you worked through to get the hang of it?