This document discusses the use of modal and related verbs to express functions such as necessity, prohibition, permission, advice, ability, and possibility in the present, past, and future tenses. It provides examples of how to use must, have to, need to, mustn't, can't, may not, should, ought to, can, may, could, be able to and will have to/need to in sentences. The document also notes that have to and must are often interchangeable, and that be able to replaces can in non-present tenses to discuss ability.

1. Modal and related
verbs

2. Function Present Past Future
necessity must,
have (got) to,
need to
had to,
needed to
will have to,
will need to
prohibition can't,
mustn't,
may not
couldn't,wasn't,
weren't allowed
won't be allowed,
will be forbidden
permission can, may could will be able to
advice should,
ought to

3.  We use must when the obligation comes from the speaker:
Ex. I really must stop eating sweets.
 We use have to when the obligation comes from somebody else or
is necessary because it is a rule or a law:
Ex. You have to/need to fasten your seatbelt.
Ex. The doctor says I have to give up smoking.

4. The difference between have to and must is very
slight, however, and often they are
interchangeable.

5.  We use mustn't, can't and may not to talk about things that are not allowed:
You mustn't smoke here.
 We use should and ought to to talk about duty, or to give advice. Ought to is more
formal than should:
We should turn around now.
You ought to warn the passengers.
 We use can, could and may to ask for permission. Could and may are more polite
than can:
Can I open the window?
 Could we go back to the airport?
May I take your luggage?
 We use can and could to make requests:
Can/Could you open the door for me?
 We use can to talk about possibility:
It can be useful later.
 We use can to talk about ability in the present and could to talk about ability in the past:
Of course I can ski. I could ski before I was five

6.  To make sentences in other tenses we use be able to instead of can:
I've never been able to go abroad alone.
We won't be able to catch our flight.
I'd like to be able to travel around the world.
 We only use must in the present tense. In other tenses we use have to:
I had to remember my boarding pass.
We'll have to buy sun cream before we leave.
We didn't have to wait in the queue