Modals and Related Expressions

As Swan (Practical English Usage (Oxford University Press, 1997), explains:

"One (of the meanings of modal verbs) is to do with degrees of certainty: modal verbs can be used to say for instance that a situation is certain, probable, possible or impossible. " p. 334

Examples:

You have to have a passport when you travel to the USA.  (necessary)

You don't have to have a passport to travel to Bahia.  (not necessary)

I must go now, my class starts at 7:30 sharp. (necessary)

***I don't have to go now, my class doesn't start until 9. (not necessary)

OBS. We use don't have to or don't need to express NO NECESSITY.

Children must not drive. (prohibition)

You can get married at the age of 21 (allowed / possible) but you can't get married earlier. (not allowed / not possible)

You can dance very well I am really surprised! (ability)

When I was young I could climb all the trees of our school. (ability - past)

You should look after your clothes.  (advisable)

You'd better look after your clothes. (advisable)

You ought to look after your clothes. (advisable)

You shouldn't walk alone at night.  (not advisable)

Would you mind turning the light on? (request)

Would you turn the light on? (request)

I would rather be at the beach than at the movies. (preference)

It may rain tomorrow. (future possibility)

They said it could rain tomorrow. (future possibility)

DEDUCTION (good presentation)

*** You find a better list with examples in your book on Appendix C. (page 150)

A good table here

(Most of the links cited below have more than these modals. You may either read / practice only about them or about all of them. And this would be a bit more challenging!  :-)