It's Greek to me - do you understand why?

As I mentioned in an earlier post, Greek is not an easy language to learn and I strongly advise anyone thinking of moving to this country on a permanent basis to at least get a basic knowledge of the language.
But for Greeks English is not an easlier language either. Most (particularly young) Greeks, speak English to some degree - the younger the better the English. I know many many Greeks whose level of English I would estimate as excellent. Yet, no matter how good, Greeks have difficulty with English prepositions. i.e. "to, at, on, in" and mix these up very easily. Why?
The answer is simple, Greek has one preposition "se" which means "to, at, on, in". On the beach is "se", in the town is "se", "to the restaurant is "se". So it is extremely difficult to a Greek to understand why we say "in the town" and not "at the town", "I saw it on television" Not "I saw it in television","I am on the island of Paxos" and not "at the island of Paxos". Indeed can you say or explain why we say in English "at the beach" and not "in the beach" or "I am in the mountains" and not "I am at the mountains"?