Prepositions are short words (on, of, to) which doesn't mean that they are easy to grasp. It is likely that learners of all levels find them difficult since you cannot always literary translate them (e.g. at). One more problem is that there are hardly any rules as to when to use which preposition. The only way to learn them is by looking them up in a dictionary, reading and learning collocations. This worksheet may be useful for both teachers and students when revising prepositions in English.