Can / Could

Can



"Can" is one of the most commonly used modal verbs in English. It can be used to express ability or opportunity, to request or offer permission, and to show possibility or impossibility.

Examples:

I can ride a horse. ability
We can stay with my brother when we are in Paris. opportunity
She cannot stay out after 10 PM. permission
Can you hand me the stapler? request
Any child can grow up to be president. possibility

Using "Can" in Present, Past, and Future

Most modal verbs behave quite irregularly in the past and the future. Study the chart below to learn how "can" behaves in different contexts.

1- Can: General ability

Present:
I can speak Chinese.
I can't speak Swahili.

Past: SHIFT TO "COULD"
I could speak Chinese when I was a kid.
I couldn't speak Swahili.

Future: SHIFT TO "BE ABLE TO"
I will be able to speak Chinese by the time I finish my course.
I won't be able to speak Swahili.

You can also use "be able to".


2- Can: Ability during a specific event

Present:
With a burst of adrenaline, people can pick up cars.
Even with a burst of adrenaline, people can't pick up something that heavy.

Past: SHIFT TO "BE ABLE TO"
With a sudden burst of adrenaline, he was able to lift the car off the child's leg.
Even the weight lifter, couldn't lift the car off the child's leg.

Future: SHIFT TO "BE ABLE TO"
With a sudden burst of adrenaline, he will be able to lift the car.
Even three men working together won't be able to lift the car.

You can also use "be able to".

3- Can: Opportunity

Present:
I have some free time. I can help her now.
I don't have any time. I can't help her now.

Past: SHIFT TO "BE ABLE TO"
I had some free time yesterday. I was able to help her at that time.
I didn't have time yesterday. I wasn't able to help her at that time.

Future:
I'll have some free time tomorrow. I can help her then.
I won't have any time later. I can't help her then.

You can also use "be able to".


4- Can: Permission

Present:
I can drive Susan's car when she is out of town.
I can't drive Susan's car when she is out of town.

Past: SHIFT TO "BE ALLOWED TO "
I was allowed to drive Susan's car while she was out of town last week.
I wasn't allowed to drive Susan's car while she was out of town last week.

Future:
I can drive Susan's car while she is out of town next week.
I can't drive Susan's car while she is out of town next week.

You can also use "may".

5- Can: Request

Can I have a glass of water?
Can you give me a lift to school?
Can't I have a glass of water?
Can't you give me a lift to school?

Requests usually refer to the near future.

You can also use "could" or "may".

6- Can: Possibility, impossibility

Anyone can become rich and famous if they know the right people.
Learning a language can be a real challenge.
It can't cost more than a dollar or two.
You can't be 45! I thought you were about 18 years old.

This use is usually a generalization or a supposition.

You can also use "could".



Could



"Could" is used to express possibility or past ability as well as to make suggestions and requests. "Could" is also commonly used in conditional sentences as the conditional form of "can."

Examples:

Extreme rain could cause the river to flood the city. possibility
Nancy could ski like a pro by the age of 11. past ability
You could see a movie or go out to dinner. suggestion
Could I use your computer to email my boss? request
We could go on the trip if I didn't have to work this weekend. conditional

Using "Could" in Present, Past, and Future

Most modal verbs behave quite irregularly in the past and the future. Study the chart below to learn how "could" behaves in different contexts.

1- Could: Possibility

Present:
John could be the one who stole the money.
Mary couldn't be the one who stole the money.

Past:
John could have been the one who stole the money.
Mary couldn't have been the one who stole the money.

Future:
John could go to jail for stealing the money.
Mary couldn't possibly go to jail for the crime.

You can also use "might" or "may".


2- Could: Conditional of can

Present:
If I had more time, I could travel around the world.
Even if I had more time, I couldn't travel around the world.

Past:
If I had had more time, I could have traveled around the world.
Even if I had had more time, I couldn't have traveled around the world.

Future:
If I had more time this winter, I could travel around the world.
Even if I had more time this winter, I couldn't travel around the world.

3- Could: Suggestion

NO PRESENT FORM
NO NEGATIVE FORMS

Past:
You could have spent your vacation in Hawaii.

Future:
You could spend your vacation in Hawaii.

4- Could: Past ability

I could run ten miles in my twenties.
I could speak Chinese when I was a kid.
I couldn't run more than a mile in my twenties.
I couldn't speak Swahili.

"Could" cannot be used in positive sentences in which you describe a momentary or one-time ability.

Yesterday, I could lift the couch by myself. Not Correct

"Could" can be used in negative sentences in which you describe a momentary or one-time ability.

Yesterday, I couldn't lift the couch by myself. Correct

You can also use "be able to".

5- Could: Polite request

Could I have something to drink?
Could I borrow your stapler?
Couldn't he come with us?
Couldn't you help me with this for just a second?

Requests usually refer to the near future.

You can also use "can", "may" or "might".

REMEMBER: "Could not" vs. "Might not"
"Could not" suggests that it is impossible for something to happen. "Might not" suggests you do not know if something happens.

Examples:

Jack might not have the key. Maybe he does not have the key.
Jack could not have the key. It is impossible that he has the key.