To "avoid" is to keep away from something or someone. To "avoid" also means to "prevent or refrain from doing something." It does not necessarily mean to refrain from doing something *ahead of time,* just to refrain from doing it. "Avoid" is a similar word in meaning to the word "prevent," but it is used differently, depending on the context of the sentence. In other words, what the sentence is trying to say will determine which word is appropriate. To "prevent" means to be in ready for, and it also means to meet or satisfy in advance of something happening. Simply speaking, it means to act ahead of something. You *WOULD* say: "I wish to avoid seeing John." You would *NOT* say: "I wish to prevent seeing John." BUT you *COULD* say: "I wish to prevent John from seeing me." I hope this helps to clarify this point better.