There are two possibilities.
- Θα πρέπει + present tense: θα πρέπει να φύγει “he has to leave”. The structure refers to the future, but it it used to express “necessity” or “obligation”. The difference between θα πρέπει να φύγει and πρέπει να φύγει is subtle and difficult to put into words. Let’s say that the first one is kind of more polite or less certain expression of the fact of his leaving.
- Sentences like θα πρέπει + imperfect or past tense, are very different: Θα πρέπει να έφυγε does not mean “he had to leave”, it means “I presume/I guess that he left”– but without having any tangible proof or evidence.
In both cases, I suggest that the role of na is to connect the two verbs without marking any semantic function. I believe that subjunctive in MG is just another name for what I call the na-form, ie the form used when you need to refer to the lexical meaning of the verb without taking in account the grammatical meaning of the tense. A very similar analysis is proposed by Delveroudi, Tsamadou and Vassilaki in “Mood and modality in modern Greek: The particle Na”. —-Prof. Marianna Katsoyiannou