Expressing modality: must, can, maybe
πρέπει, μπορεί, ίσως, θα + past: the small machinery that separates “he must leave” from “he must have left”.
πρέπει να — obligation and deduction
Impersonal and invariable: Πρέπει να φύγω (I must leave), Έπρεπε να φύγω (I had to). With a perfect after it, it turns into deduction: Πρέπει να έχει φτάσει — he must have arrived. Unfulfilled past obligation: Έπρεπε να είχες έρθει (you should have come).
μπορώ or μπορεί?
Conjugated μπορώ = ability or permission: Μπορώ να κολυμπήσω. Frozen 3rd-person μπορεί = possibility: Μπορεί να βρέξει (it may rain). Compare: Μπορεί να έρθει (he may come) / Μπορεί να έρχεται (he may be on his way).
Conjectural θα
θα + past is not a future: it is a deduction. Θα έφυγε ήδη — he must have already left. Θα ήταν κουρασμένος — he was probably tired. Add ίσως (maybe) and μάλλον (probably) to grade it: Μάλλον θα αργήσει.
Your turn
Choose the correct form.