MODALS FOR NECESSITYAND SUGGESTIONS

Modal verbs are helping/auxiliary verbs that express ideas like ability, necessity, and prohibition.

Many modal verbs have more than one meaning. They are always followed by the simple form of a verb.

MUST

NEED TOHAVE TO

DON´T HAVE TO

HAD BETTER o 'D BETTER 

OUGHT TO 

SHOULD

SHOULDN´T

MUST We can use 'must' to show that we are certain something

is true . In general, must expresses personal obligation. 

Ex. We must show our passport at the airport

USES

HAVE TO

We often use have to to say that something is obligatory.In general, have to expresses impersonal obligation

Ex. John has to wear a tie at work. Children have to go to school

NEED TO

We can use need to say something is necessary or unnecessary 

Ex. You usually need to check in at least two hours before a flight leaves. I don’t need to take a raincoat

HAD BETTER

We use “had better” to give advice about specific situations, not general ones.Ex. You'd better tell her everything.

OUGHT TO Is used to advise or make recommendations. also expresses assumption or expectation as well as strong probability.Ex. You ought to stop smoking. RECOMMENDATION This stock ought to increase in value. PROBABILITY

SHOULD We often use should  when offering advice or opinions .

Ex. You should see the new James Bond movie. It's great!.People should worry more about global warming.

http://web2.uvcs.uvic.ca/elc/studyzone/330/grammar/330-modals-necessity1.htm