CAN, CAN’T, COULD, COULDN’T

What’s the difference between CAN and COULD?

CAN and COULD are very similar in the English language. Both words are very common because they have many meanings or functions. This means that both CAN and COULD can be used in various ways and different situations.

 

CAN	COULD
FRIENDLY FORM	POLITE FORM
ABILITY	ABILITY IN THE PAST
PERMISSION	PERMISSION
REFUSE PERMISSION	REFUSE PERMISSION
REQUEST	REQUEST
OFFER	OFFER
POSSIBILITY	POSSIBILITY
IMPOSSIBILITY	IMPOSSIBILITY

How are CAN and COULD the same?

You can use both can or could for REQUESTS and OFFERS. The difference between the two is very small but could is more polite than can. Compare the following example sentences.

REQUESTS
Can you help me? Yes, no problem.	Could you help me? Yes, of course.
Can you turn on the light, please?	Could your turn on the light?
OFFERS
Can I help you?	Could I help you?
I can carry your bag for you.	I could carry your bag for you.
PERMISSION
Can I ask a queston?	Could I ask a question?
Can I have some more chocolate?	Could I have some more chocolate?

How are CAN and COULD different?

CAN	COULD
POSSIBILITY	POSSIBILITY (UNCERTAIN)
We can win.	We could win.
We can fly to Rome this weekend. The flights are cheap.	We could fly to Rome this weekend, the flights are cheap.
IMPOSSIBILITY	IMPOSSIBILITY
It can’t be true. I don’t believe it.	– – — – — – — – — – — – –
It can’t snow in the Sahara. It’s too hot.

When we use CAN to say that something is possible, it is more certain than COULD (i.e. could = maybe).

NOTE: Use CAN’T not COULDN’T to say that something is impossible in the present.

CAN	COULD
ABILITY	ABILITY IN THE PAST

We use CAN to say that we know how to do something, i.e. for a skill, knowledge.

Examples: I can ride a horse. / I can play the piano.

NOTE that COULD is only the past of CAN when it is used for ABILITY.

Examples: I could run very fast when I was young. / I could walk when I was only 1 year old.

NOTE we do not use COULD to say that we did something once or one time only. To express this, you need to use other words.

Examples: I was able to pass the exam last year. / I managed to pass the exam last month. Not: I could pass the exam last week.

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