1. General Revision Modal Verbs

2. What are modal verbs?
Modals (also called modal verbs, modal auxiliary verbs, modal auxiliaries) are special verbs which are different from normal verbs like "work, play, visit...“
They give additional information about the function of the main verb that follows it.

3. Use of modal verbs: Express functions such as: Permission Ability
Obligation
Prohibition
Lack of necessity
Advice
possibility
probability

4. Modal verbs are followed by an infinitive without "to“:
Examples:
You must stop when the traffic lights turn red.
You look awful! You should see a doctor.
Exception:
You ought to go to the doctor.

5. must must not can could may might
Modal Verb
Meaning
Expressing
Example
must
to have to
100 % obligation
I must stop when the traffic lights turn red.
to be very probable
logical conclusion (deduction)
He must be very tired after such enormous work
must not
not to be allowed to
prohibition
You must not smoke in the hospital.
can
to be able to
ability
I can swim
to be allowed to
permission
Can I use your phone please?
it is possible
possibility
Smoking can cause cancer !
could
ability in the past
When I was younger I could stay up all night and not get tired..
more polite permission
Excuse me, could I just say something?
It could rain tomorrow!
may
May I use your phone please?
it is possible, probable
possibility, probability
It may rain tomorrow!
might
Might I use your phone please?
weak possibility, probability
I might come and visit you in America next year, if I can save enough money.

6. need need not should/ought to had better
Modal Verb
Meaning
Expressing
Example
need
necessary
necessity
Need I say more?
need not
not necessary
lack of necessity/absence of obligation
I need not buy any tomatoes. There are plenty in the fridge.
should/ought to
used to say or ask what is the correct or best thing to do
50 % obligation
I should / ought to see a doctor. I have a terrible headache.
to suggest an action or to show that it is necessary
advice
You should / ought to revise your lessons
to be very probable
logical conclusion (deduction)
He should / ought to be very tired after such enormous work
had better
You 'd better revise your lessons

7. GOLDEN RULES ABOUT MODAL VERBS
Modal verbs never add an -s in the third person singular: 
He must work harder to pass his exams
Modal verbs don’t need auxiliaries to make negative sentences or questions: 
Should I apologise?
I mustn´t smoke in hospitals
Modal verbs only have one form. You need other verbs to express the same thing in the past, future etc.
I´ve been able to finish on time.  

8. 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.  

9. EXPRESSING OBLIGATION OR NO OBLIGATION
We use must and have to to express obligation.
The difference between them is:  We use must when the obligation is inside the speaker. It is a moral obligation.
We use have to when the obligation is outside the speaker. It comes from the situation.

10. EXPRESSING OBLIGATION OR NO OBLIGATION
I must do some exercise to get fit.
We have to be quiet in class. That’s the rule.
Note: Have to is not a modal verb. We use don’t / doesn’t have to, needn’t and don’t /doesn’t need to to express no obligation.
You don’t need to get up early. It is Sunday!
She doesn’t need to wear a coat. It is warm outside.
She needn’t cook tonight. We´re eating out.  

11. EXPRESSING PROHIBITION
We use mustn’t to express prohibition.
Children under 16 mustn’t go in there. At school we mustn’t smoke.