At which point in time did the car crash? I gave the dog plenty of milk every day, for which reason it soon grew fat. That is a question to which there is no answer. There are ten people on the train, two of which are murderers. A good question, Hoorman. My advice is to go to tatoeba.org, type ("in quotes"): "at which" "to which" "for which" "of "which" under english to farsi, or just english to [any language], then study the patterns and practicing using some of them. This is an organic way to absorb a language.