Same prepositional hazard!
I agree with Mistica, this preposition thing--dangerous. I mean: for, from, of, etc. are all words that relate the noun to the sentence. But, it is so hard to explain to the student which preposition is appropriate and why. Also, the placement of the preposition is kind of difficult to explain. Another thing, the mixing of the words at and on (also prepositions). These might be easier to explain, but these specifically are an obvious problem that I've been encountering over and over again.