Modal verbs to express obligation, lack of obligation, prohibition and advice

1. MODALS OF OBLIGATION
MODAL VERBS

2. What are MODAL VERBS?

3. MUST & HAVE TO
• I must finish my homework before 8
• We have to wear a uniform
They are both used to express
that something is necessary

4. MUST
• I must find a hotel to stay at tonight
• The speaker decides that something is
necessary (INTERNAL decision)

5. HAVE TO
• I have to wear a uniform
• Somebody else has made the decision,
it’s not the speaker’s opinion
(EXTERNAL decision)

6. OTHER TENSES
• MUST: past & future with HAVE TO
• When I was young, we had to wear a
uniform at school
• Next year you will have to study harder

7. MUSTN’T & DON’T HAVE TO
• Very different meanings in negative:
• You mustn’t use a dictionary
• You don’t have to use a dictionary

8. MUSTN’T
• You mustn’t use a dictionary.
• (It’s forbidden /prohibited)
• =can’t

10. DON’T HAVE TO
• You don’t have to use a dictionary. This
exercise is really easy
• (It’s not necessary)

11. SHOULD / SHOULDN’T
• I think you should see the doctor
• I think you shouldn’t watch so much TV
• We use SHOULD/ SHOULDN’T
to give some advice