They are quite different and I'm not sure how I can explain but I could just give some exams sentences 1. I will do it for you. 2. Here is a present for your brother. "For" is when a action is done towards someone 1. May I please have a slice of cake? 2. There was a decrease of 10% "Of" talks about the relation between a whole object and a smaller part of it This is quite difficult to explain but I hope this helped.