Modal Verbs

Modals  are special verbs which behave irregularly in English.  They are used to indicate modality. They give additional information about the function of the main verb that follows it .They have a great variety of communicative functions. 

Modal verbs are used to express functions such as:

1. Permission
2. Ability
3. Obligation
4. Prohibition
5. Lack of necessity
6. Advice
7. possibility
8. probability

Examples:

• You must stop when the traffic lights turn red
• You should see to the doctor
• There are a lot of tomatoes in the fridge. You need not buy any.

Exception:

• You ought to go to the doctor

       Here is a list of  modals 

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	to be able to	ability in the past	When I was younger I could stay up all night and not get tired..
	to be allowed to	more polite permission	Excuse me, could I just say something?
	it is possible	possibility	It could rain tomorrow!
may	to be allowed to	permission	May I use your phone please?
	it is possible, probable	possibility, probability	It may rain tomorrow!
might	to be allowed to	more polite permission	Might I use your phone please?
	it is possible, probable	weak possibility, probability	I might come and visit you in America next year, if I can save enough money.
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	to suggest an action or to show that it is necessary	advice	You 'd better revise your lessons