Modal verbs are used to express ideas like obligation, lack of obligation, prohibition, and advice. They behave differently than regular verbs in that they do not take -s in the third person singular present and use "not" for negation rather than tense. Common modal verbs are must, have to, should, and ought to. Must expresses strong obligation or advice while have to expresses external obligation. Don't have to expresses lack of obligation. Mustn't is used for prohibition while should and ought to provide recommendations. Have got to is also used in spoken English like must and have to.

1. Obligation and
advice
Intermediate 2

2. Modal verbs are auxiliary verbs that provide additional
and specific meaning to the main verb of the sentence.
They are special verbs which behave very differently from
normal verbs.
Here are some important differences:

3. Here are some important differences:
1. Do modal verbs take the –s in the 3rd singular person in
present simple?
2. How de we make a modal verb negative?
3. Can modal verbs take a future or past form?

4. Modal verbs do not take "-s" in
the third person:
He can speak Chinese.
She should be here by 9:00.

5. You use "not" to make modal verbs negative,
even in simple present and simple past.
He should not be late.
They might not come to the party.

6. Many modal verbs cannot be used in
past or future tenses.
She musted study very hard
not correct

7. Obligation Lack of
obligation
Prohibition Advice
?? ?? ?? ??
?? ??

8. Obligation Lack of
obligation
Prohibition Advice
Have to ?? ?? ??
Must ??

9. Obligation Lack of
obligation
Prohibition Advice
Have to don’t have to ?? ??
Must ??

10. Obligation Lack of
obligation
Prohibition Advice
Have to don’t have to Mustn’t ??
Must ??

11. Obligation Lack of
obligation
Prohibition Advice
Have to don’t have to Mustn’t Should
Must Ought to

12. Expressing obligation
or no obligation
Must vs. have to (+ infinitive)

13. Compare…
I must finish the report by 7.00.
You have to finish the report by
7.00!

14. Have to or must?
We use when the necessity comes from the speaker.
We use when the necessity comes from outside the
speaker, i.e. rules or laws.

15. Must (+ infinitive)
• We usually use must when the
obligation is inside the speaker (it is a
moral obligation).
I must do some exercise to get fit
have to (+ infinitive)
• We use have to when the obligation
is outside the speaker (it comes from
the situation).
I have to take this book back to the library.
We have to be quiet in class. That´s the
rule.
We use must and have to to express obligation. The
difference between them is:

16. Have to or must?

17. …
Compare…
…

18. You have to tidy your room!
Compare…
I must tidy my room!

19. I’ve got to go now!
It’s very late!
Have got to (+ infinitive)
Have got to is often used instead of
must or have to in spoken English.

20. Expressing no obligation
• To express lack of obligation we use:
don´t / doesn´t have to + infinitive
You don´t have to get up early. It is Sunday!
She doesn´t have to wear a coat. It is warm outside.
We don’t need to cook tonight. We´re eating out.
Have to is not a
modal verb

21. Expressing prohibition
• We use mustn’t to express prohibition.
Children under 16 mustn’t go in there.
At school we mustn’t smoke.

22. Expressing prohibition
• We can often use can’t or be not allowed to instead of
mustn’t.
Children under 16 mustn’t go in there.
At school we can’t smoke.
We’re not allowed to park here.

23. Expressing advice
Should and ought to (+ infinitive)

24. should and ought to (+ infinitive)
You should read more if you really
want to improve your level.

25. Expressing advice
• We use should and ought to to express advice or make
recommendations:
You should eat less fat if you want to lose weight.
You ought to relax if you don´t want to get ill.
You shouldn´t be rude to anybody.
You oughtn´t to tell lies.

26. Expressing advice
• We can use must and have to give strong advice when we
think it’s very important that someone does something
If you feel really ill, you must go to the doctor’s
When you’re in Venice, you have to have a drink at Harry’s bar!