Luluk, This would be easier to answer if you had some examples. With the verb "happen" here are some examples: Within - defined with a beginning and an end "this needs two happen within those two hours" Due - a specific time "this needs to happen, it is due at 4:00" During - two things at the same time, the beginning and end may not be defined "this needs to happen during the party" For - the reason something is happening "this needs to happen for John to be happy"