This document discusses modal verbs used to express obligation or necessity. It explains that must and have to are both used to express something is necessary, with must indicating an internal decision and have to indicating an external decision. It also discusses the differences between mustn't and don't have to in the negative, with mustn't meaning something is forbidden and don't have to meaning something is not necessary. Finally, it notes that should and shouldn't are used to give 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