modals

ability (=ικανότητα): I can swim. (Past simple: could Future Simple: I will be able to)

asking for/giving permission (=ζητώ/δίνω την άδεια για κάτι): Can I go out? Yes, you can.

possibility (=πιθανότητα). The job of the lifeguard can be dangerous.

asking for /giving permission (formal=επίσημο): May I go out? Yes, you may.

possibility: It may rain tomorrow. (Πιθανόν να βρέξει αύριο)

obligation(=υποχρέωση) Ι must study hard for the test. We mustn't hit people.

strong probability (μεγάλη πιθανότητα). The lights are on. He must be in the house. (Τα φώτα είναι αναμμένα. Είναι σίγουρα στο σπίτι)

Be careful! The lights are off. He can't be in the house. (Όταν θέλουμε να πούμε ότι κάτι σίγουρα δεν συμβαίνει, χρησιμοποιούμε can't)

advice(=συμβουλή). If you feel ill, you should visit the doctor. You shouldn't eat too many sweets.

Ι have to finish my homework (because my mum says so!) Do I have to visit your friend? (Είναι απαραίτητο να πάμε επίσκεψη στη φίλη σου;)

Be careful! I don't have to=δεν είναι απαραίτητο. Ι don't have to study today because tomorrow is a holiday!