English Grammar - Tagged Questions

• What is the difference between past simple and past perfect?
• Learning English I constantly mix up the prepositions 'of' and 'for'. Could you explain the core difference in their usage with clear examples focusing on possession purpose and common phrases?
• I constantly mix up the usage of "in which" "of which" "at which" and "to which". Can you clarify the grammatical rules and provide examples for when to correctly use each of these relative pronoun phrases?