Modal auxiliaries like can, could, may, might, and should are used with main verbs to express meanings like ability, permission, possibility, advice, and recommendation. Modals don't have endings for tense and don't take infinitives or auxiliary verbs like do/does/did. Negation is formed by placing "not" after the modal.

1. Modal auxiliaries
Modals help the main verb express special
meanings, like ability, permission, etc.
 Modals don’t have an -s, -ed, or -ing ending.
o He can drive (NOT: He cans drive)
 Don’t use an infinitive with modals
o He can go. (NOT: He can to go)
 Don’t use the auxiliaries “do”, “does” or “did”
o Can you swim? (NOT: Do you can swim?)
 To form the negative, put NOT after the modal
o He should not go.

2. Examples
We can play football.
We could play football.
We may play football.
We might play football.
We must play football.
We mustn't play football.
We have to play football
We ought to play football.
We shall play football.
We should play football.
We would play football.

3. Can
CAN
can = ability (+) I can speak English.
I know how to do it. (-) I can't speak French.
I'm able to do it. (?) Can you speak Arabic?
can = present/future possibility (+) The doctor can see you tomorrow.
It is/will be possible. (-) I can't buy a car yet. I need more
money.
(?) Can you buy a hammer at the
supermarket?
can = informal permission
You have permission to do it. (+) You can have a cookie after dinner.
Is it OK if I do it? (-) You can't go to the movies tonight.
(?) Can I go to the park?

4. Could
COULD
(+) I could ride a bicycle when I was five
could = past ability
years old.
I knew how to do it in the past.
(-) I couldn't drive a car at age 7
I was able to do it in the past.
(?) Could you skate when you were young?
(+) It could rain tomorrow.
could = present & future (-) It couldn’t snow in Hawaii
possibility It is/will be possible. (?) Could you buy some milk on your way
home?
(+) I could walk to school alone when I was 7
could = permission years old.
You have/had permission to do it. (-) I couldn't stay up late when I was young.
Is it OK if I do it? (?) Could I have a cookie?

5. May
MAY
may = present & future possibility
(+) The store may be closed
It is/will be possible.
(-) The store may not be closed
There is/will be an opportunity for it.
(+) You may eat dessert after
may = permission dinner
You have/will have permission to do it. (-) You may not eat dessert after
Is it OK if I do it? dinner
(?) May I eat dessert after dinner?

6. Might
MIGHT
Might = present & future
possibility (+) The store might be closed
It is/will be possible. (-) The store might not be closed
There is/will be an opportunity for it.

7. Should
SHOULD
Should = present & future (+) You should take some
advice or recommendation medicine
It is/will be a good idea (-) You shouldn’t drink too
It is/will be a good suggestion. much
(?) What should I wear?
The use of “maybe” with “should”, softens the advice. Compare:
You should eat healthier food (you are giving a direct advice)
Maybe you should eat healthier food (you are giving an option)