CAN, COULD (ABILITY)

CAN 

Can is an auxiliary verb, a modal auxiliary verb. We use can to:

    talk about possibility and ability

    make requests

    ask for or give permission

Structure of Can

subject + can + main verb

The main verb is always the bare infinitive (infinitive without "to").

	subject	auxiliary verb	main verb
+	I	can	play t ennis.
-	He	cannot	play  tennis.
		can't
?	Can	you	play  tennis?

Notice that:

    Can is invariable. There is only one form of can.

    The main verb is always the bare infinitive.

The main verb is always the bare infinitive (infinitive without "to"). We cannot say:

Use of Can

can: Possibility and Ability

We use can to talk about what is possible, what we are able or free to do:

    She can drive a car.

    John can speak Spanish.

    I cannot hear you. (I can't hear you.)

    Can you hear me?

Normally, we use can for the present. But it is possible to use can when we make present decisions about future ability.

    Can you help me with my homework? (present)

    Sorry. I'm busy today. But I can help you tomorrow. (future)

COULD 

Could is an auxiliary verb, a modal auxiliary verb. We use could to:

    talk about past possibility or ability

    make requests

Structure of Could

subject + could + main verb

The main verb is always the bare infinitive (infinitive without "to").

Subject	auxiliary verb	main verb
+           My grandmother	could	swim.
-            She	could not	walk.
	couldn't
?           Could	your grandmother	swim?

Notice that:

    Could is invariable. There is only one form of could.

    The main verb is always the bare infinitive.

The main verb is always the bare infinitive. We cannot say:

Use of Could

could: Past Possibility or Ability

We use could to talk about what was possible in the past, what we were able or free to do:

    I could swim when I was 5 years old.

    My grandmother could speak seven languages.

    When we arrived home, we could not open the door. (...couldn't open the door.)

    Could you understand what he was saying?

We use could (positive) and couldn't (negative) for general ability in the past. But when we talk about one special occasion in the past, we use be able to (positive) and couldn't (negative). Look at these examples:

               

	Past
	General	Specific Occasion
+	My grandmother could speak Spanish.	A man fell into the river yesterday. The police were able to save him.
-	My grandmother couldn't speak Spanish.	A man fell into the river yesterday. The police  able to save him.