Which is the most difficult grammar issue for you to really understand?

I've been studying English since I was a child and I'm still not completely secure about using Present Perfect (which does not exist in my own language). I know the general rules, I can deal with them skillfully , but there are times I get confused, and I still haven't found the ultimate explanation that would reassure me. The same for future tenses, though this topic is a bit easier.

Aspect in Russian. It's diabolically difficult. I own the best English language based grammar for the Russian language. It's 630 pages long and a full 50 pages of it is devoted to aspect.