They never change their form. You can't add "s", "ed", "ing"...
They allow speakers to express certainty, possibility, willingness, obligation, necessity, ability
present tense: modal + infinitive (without "to")
e.g. I can run fast
past tense: modal + have + past participle, had
can --> could, will --> would
e.g. He must have been rich,
he must leave --> he had to leave