This document discusses the use of various modal verbs to express obligation, prohibition, absence of obligation, and advice. It explains that "must" expresses strong obligation or necessity, while "have to" expresses external obligation. "Mustn't" and "can't" are used to express prohibition. "Don't have to" and "needn't" express an absence of obligation. "Should" and "ought to" are used to give advice, while "had better" suggests there may be problems if something is not done. The past forms like "should have" are used to give advice about past actions.

1. OBLIGATION/ PROHIBITION/
ABSENCE OF OBLIGATION/
ADVICE

2. MUST/ HAVE TO : STRONG OBLIGATION / NECESSITY.
In the first person,
Must expresses “moral obligation”
- I must write to Ann, I haven’t written to her for ages. (The speaker
thinks it’s necessary. It’s his/her moral obligation)
Compare it with “have to”:
- I have to wear a uniform in my school (it means external obligation)
In the second person:” Must” expresses authority in the speaker:
- Doctor: You must cut down on smoking and fatty food.
- Mother: You must wear a dress tonight. You can’t go to the opera in
those dreadful jeans.
If you are talking to a friend you usually use “have to”:
- It’s already 12 o’clock. Be careful, you have to be back home early.
Have a look at some examples, in the third person:
- The government really must do something about unemployment. (The
speaker personally feels that the government must do something)
With “have to” the speaker is not giving his own feelings. He is just giving
facts.
- Ann’s eyes are not very good. She has to wear glasses.

3. For the past, future, past participle and
infinitive, we have to use “have to”
- There were no buses so she had to walk.
- You will have to study harder when you
go to university.
- I have always had to help at home.

4. PROHIBITION
Mustn’t
You mustn’t drive so fast.
People mustn’t smoke at school.
Can’t
Can’t can also express probibition
You can’t park here. (you’re not allowed to)

5. Lack / absence of obligation
DON’T HAVE TO (= NEEDN’T) expresses
absence of obligation.
- We don’t have to get up early on
Saturday mornings.
- You needn’t worry (= You don’t have to
worry).

6. • ADVICE:
We use SHOULD to give advice
- You should eat more fresh fruit.
- You shouldn’t waste so much time watching TV
- Should we talk to him?
OUGHT TO is quite similar to should, but less
frequently used.
- You ought to come more frequently.
HAD BETTER: in a specific situation. You suggest
there will be a problem if something is not done.
I have a terrible headache.
- You’d better go home.
-You’d better not cry, you’d better not pout. Santa
Claus is coming to town.

7. NOTICE THE DIFFERENCE:
ADVICE IN THE PAST.
-It rained all the time and I didn’t take my
umbrella. I should have taken my umbrella.
(it was the right thing to do but you didn’t do it)
You copied at the exam; the teacher saw you.
You shouldn’t have copied.

8. ABSENCE OF OBLIGATION IN THE PAST
-It didn’t rain at all, but I took my umbrella.
I needn’t have taken my umbrella.
(you did something which wasn’t necessary)
- I studied a lot, but the exam was very easy.
I needn’t have studied so hard.

9. - I spent my holidays in sunny Spain so I
didn’t need to take my umbrella.(=didn’t
have to)
(it wasn’t necessary/ absence of obligation
and I didn’t do it.)
- I spent my holiday in rainy England so I
had to take my umbrella.
(obligation in the past)

10. SOME MORE EXAMPLES:
Last year maths was very easy, so I didn’t
have to study a lot. (lack of obligation/ I
didn’t have to and I didn’t do it)
Last year maths was very difficult, so I had to
study a lot. (obligation in the past / I had
to do something and I did)

11. The exam was very difficult and I didn’t study
hard. I should have studied harder. (It
was the right thing to do, but you didn’t do
it)
I studied very hard; but the exam was
extremely easy. I needn’t have studied so
hard. (you did something which wasn’t
necessary)