Can I use " of" instead of for? In the sentence " One reason for the trip is for business". Can I use " one reason of the trip" instead of " one reason for the trip"? If not, why?

Some questions about " over", " by", " for" I am really confused how to use " by" , " for", " over", " of": 1. " The meeting in China were delayed by three days". In this sentence, why use " by", not " for"? 2. " As a result, he had to change the dates for his stay in China". In this sentence, why use " for', not " of"? 3. " One reason for the trip was for business". Why use " for', not"of"? 4. "This tree is bending over because of the strong wind." Why use " bending over" not just " bending"? 5." It is going to be windy for the new next few days." Why use " for", not "in"? Thanks in advance.

Before leaving on the trip, Paul has several problems. Why it is not " before leaving home, Paul has several problems."

why the sentence use " left" instead of " leave"? A sentence on " Oxford English Dictionary" is " Come on, it is time we left". Why it use left, not leave?