Modal verbs and their meaning

Modal verbs:

• They never change their form. You can't add "s", "ed", "ing"...
• They are always followed by an infinitive without "to" (e.i. the bare infinitive.)
• They are used to indicate modality allow speakers to express certainty, possibility, willingness, obligation, necessity, ability

Examples:

I can swim. (Ability)

Can I use your phone please? (Permission)

Can we visit Grandma at the weekend? (Suggestion)

It can get very hot in Arizona. (Possibility)

When I was younger I could run fast. (Ability in the past)

Excuse me, could I just say something? (Polite permission)

May I help you? (Polite suggestion)

You should / ought to revise your lessons. (Advice)