Prepositions are an important part of the English language and they are everywhere! They are in prepositional phrases, phrasal verbs, idioms and expressions. They can go with verbs, adjectives or nouns and then we also have prepositions for time and place. Greek learners- and not only them,I am guessing- find it really hard to remember where to put it and if they remember that they need a preposition, they may still have problems in finding which one they should use.
A problem with prepositions is that they usually cannot be translated especially if you try word-to-word translation - something Greek students try more often than needed- contrary to our advice not to do so! So most of the times, the choice of the correct preposition is not something they can logically correlate with their mother tongue knowledge.
So, how can prepositions be taught? There is actually no rule or pattern that we can teach our students. They have to learn most of them by heart. What is very helpful, however, is to learn to recognise and spot a preposition when they see one. To do a lot of gap-filling exercises with prepositions. To read different kinds of texts mostly long ones. Literature, articles, and, if they prefer the internet, blogs.
One might argue, though,that prepositions are not that important! Especially nowadays that there is a shift towards the communicative function of the language. But I believe that a sense for prepositions is essential for learners that wish to acquire a sense for the language. In my opinion, especially when we talk for levels B2 and upwards, the appropriate usage of prepositions can distinguish a competent learner from a fluent one. But, you can't start teaching prepositions at level B2, it is knowledge that has to be built slowly starting from year 1 of learning English. We should make our students aware that prepositions are as significant as tenses-talking about Greek teachers, I know our obsession with tenses-and the sooner they realize it the better.