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? You technically can, but it sounds awkward. It's because "for the trip" conveys more information than "of the trip". When you say "of (something)", you are only associating those two concepts. When you say "for (something)", you are associating the two concepts and also emphasizing that the first concept affects the other in a strong way. "I made this for you." versus "I made this of you." Both of those are correct, but one is more awkward because it conveys less useful information. The first example gives more information by linking "you" and "this" while also saying that "this" affects "you" in a strong way. The second example only links the two concepts "this" and "you" without saying if one affects the other in any particular way.