Can and could are cousins. Can covers the present: ability, permission, possibility and requests. Could covers the past, softens requests, and adds a note of uncertainty to the present.
The pair hides one real trap. Could describes general ability in the past ("I could swim at five"), but a single past achievement needs was able to or managed to: "The climbers managed to reach the top." "Could reach" would only say the ability existed, not that it was used successfully.
This page separates the four jobs of can, the four jobs of could, and the exception, with a 20-question drill that mixes all of them.
The rule
Can: present ability
- Use can for a skill or ability now: "I can swim", "She can speak three languages."
- Negative: cannot or can't. "He cannot drive" means he has no driving skill.
- Question: "Can you play the piano?" The short answer repeats can: "Yes, I can."
- Can has no third-person -s: "She can swim", not "She cans swim".
Can: permission and requests
- Use can to ask for and give permission: "Can I use your phone?" "You can take the day off."
- The negative cannot shows that permission is refused: "You cannot park here."
- Can makes a simple, everyday request: "Can you hold the door, please?"
- Can is neutral and friendly. Could is the more polite version of the same request.
Can: possibility
- Use can for things that are generally possible: "Exercising can help you sleep better."
- Negative can't shows something is impossible: "That can't be right."
- For a specific future possibility, could is more usual: "It could rain later."
- Can and could never take to: "I can swim", not "I can to swim".
Could: the past and the polite present
- Use could for general past ability: "I could read before I started school."
- Negative: could not or couldn't. "She couldn't ride a bike until she was nine."
- Use could for polite requests: "Could you pass me the salt, please?"
- Use could for present or future possibility with uncertainty: "She could be at the office, but I am not sure."
- Could also reports past permission: "The teacher said we could leave early."
The exception: one past success takes was able to
- Could describes the ability, not the completed act: "I could run fast when I was young."
- For one successful achievement on one occasion, use was able to or managed to: "She was able to finish the race despite the rain."
- The negative couldn't works for single events too: "I couldn't finish the report yesterday."
- For other tenses, use be able to: "I will be able to help tomorrow", "I have been able to rest this week."
Examples
- I could swim when I was five.General ability in the past takes could.
- She can speak three languages fluently.Present ability takes can.
- Could you pass me the salt, please?A polite request uses could.
- It could rain later, so take a coat.A future possibility with uncertainty takes could.
- Can I borrow your pen?Everyday permission uses can.
- She said I could borrow her bike.Reported permission uses could.
- I cannot hear you. The line is bad.Present inability uses cannot.
- When we were children, we could play outside until dark.Past permission takes could.
- Yesterday she was able to finish the race despite the rain.A single past achievement takes was able to, not could.
- After the surgery, he was able to walk again.One successful recovery takes was able to.
Common mistakes
- Wrong: She cans swim very well. Right: She can swim very well.
- Modal verbs never take the third-person -s. Can stays can for every subject.
- Wrong: I can to play the guitar. Right: I can play the guitar.
- Can is followed by the base verb without to.
- Wrong: The climbers could reach the top yesterday, so they celebrated. Right: The climbers were able to reach the top yesterday, so they celebrated.
- One successful achievement needs was or were able to. Could would describe the general ability, not the completed climb.
- Wrong: Could you to open the window? Right: Could you open the window?
- After could, use the base verb with no to.
- Wrong: When I was young, I could to run very fast. Right: When I was young, I could run very fast.
- Could is followed directly by the base verb.
Practice drill
Answer all 20 questions. Each answer is marked at once, and the rule is explained under the question.
I ___ swim when I was five.
Show answer and explanation
Answer: A. could General ability in the past takes could: I could swim.
She ___ speak three languages fluently.
Show answer and explanation
Answer: B. can Present ability takes can, and can never takes an -s: She can speak.
When I was a child, I ___ not ride a bike.
Show answer and explanation
Answer: C. could Past inability uses could not: I could not ride a bike.
___ you pass me the salt, please?
Show answer and explanation
Answer: D. Could A polite request uses could: Could you pass me the salt?
Yesterday she ___ finish the race despite the rain.
Show answer and explanation
Answer: A. was able to A single past achievement takes was able to, not could.
It ___ rain later, so take a coat.
Show answer and explanation
Answer: B. could A specific future possibility with uncertainty takes could.
___ I borrow your pen?
Show answer and explanation
Answer: C. Can Everyday permission uses can: Can I borrow your pen?
She said I ___ borrow her bike.
Show answer and explanation
Answer: D. could Reported permission takes could: She said I could borrow her bike.
I ___ hear you. The line is bad.
Show answer and explanation
Answer: A. cannot Present inability uses cannot: I cannot hear you.
When we were children, we ___ play outside until dark.
Show answer and explanation
Answer: B. could Past permission takes could: we could play outside.
He ___ run ten kilometres when he was younger.
Show answer and explanation
Answer: C. could General past ability takes could, and the base verb follows without to.
Sorry, I ___ attend the meeting yesterday.
Show answer and explanation
Answer: D. could not Past inability on a specific day uses could not, which works for single events.
You ___ use my phone if you need to.
Show answer and explanation
Answer: A. can Giving permission now uses can: You can use my phone.
She ___ be at the office now, but I am not sure.
Show answer and explanation
Answer: B. could Present possibility with uncertainty takes could: She could be at the office.
At that time, I ___ not understand the problem.
Show answer and explanation
Answer: C. could Past inability takes could not: I could not understand.
We ___ see the mountains from our hotel.
Show answer and explanation
Answer: D. could Past ability of perception takes could: We could see the mountains.
I ___ finish the report yesterday, so I stayed late.
Show answer and explanation
Answer: A. was able to One completed achievement takes was able to, not could.
___ you lend me your umbrella?
Show answer and explanation
Answer: B. Could A polite request uses could: Could you lend me your umbrella?
My grandfather ___ read without glasses.
Show answer and explanation
Answer: C. could General past ability takes could: my grandfather could read.
After the surgery, he ___ walk again.
Show answer and explanation
Answer: D. was able to A single successful recovery takes was able to, not could.
Questions learners ask
Is "Can I...?" or "May I...?" more polite?
May I is the most formal and is common in writing and in careful speech: "May I ask a question?" Can I is friendly and everyday. Could I is a polite middle choice. For everyday permission, can is never rude. For a formal setting, may is the careful choice.
Why is "could" wrong for one past success?
Could describes an ability that existed, not an act that was completed. "I could run a marathon" says the ability was there. "I was able to run a marathon last year" reports that the run actually happened. For a single completed achievement, use was able to or managed to. The negative couldn't works in both cases.
How do I make questions about ability in the past?
Use could before the subject: "Could you swim when you were five?" For one specific occasion, use were you able to: "Were you able to finish the report yesterday?" The answer can use could for general ability and was able to for the specific success.
More grammar topics
Gerund vs infinitive
Verb plus -ing or verb plus to: the pattern lists, and the verbs where meaning flips.
Past continuous
Was or were plus -ing: the action in progress, the interruption, and the background of a story.
Past simple vs past continuous
The event or the background: choosing the right past tense, and how when and while help.
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