Can or could?
Could is not only the past of can. It is also the softer version of it, and that second job is where the choice gets interesting.
The short answer
Can states what is possible or allowed now. Could does three different things: it is the past of can, the politer way to ask, and a way of saying something is possible but not certain.
Which one do you need?
| Meaning | Form | Example |
|---|---|---|
| ability now | can | She can drive. |
| ability in the past | could | She could drive at 17. |
| asking, neutral | can | Can you help? |
| asking, politer | could | Could you help? |
| possible, not certain | could | It could rain. |
Practice the choice
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Getting your first question ready.
Side by side
Three sentences for each form, taken from the 1,077 sentences this drill uses. The answer is highlighted.
can
- We swim here every summer, so we can swim in the ocean without fear.
- The artist works outdoors every day, and she can paint beautiful landscapes.
- There has been a guitar in the house since Christmas; can she play the guitar yet?
could
- If we got stuck on the last question, could we ask the teacher for help?
- If the lifeguard were on duty, could we swim in the deep end of the pool?
- On opening night, the actor could perform live on stage without any mistakes.
Could as the past of can
The plain case. Whatever can says about now, could says about then:
I can read Spanish. → At school I could read Spanish.
This works for lasting ability, the kind that covers years. For one occasion when something came off, English switches to was able to or managed to:
The lock was stiff, but I managed to open it. — not I could open it
The negative has no such restriction. Couldn’t is fine for both: I couldn’t open it.
Could as the polite can
Both ask for the same thing; could asks more gently:
- Can you pass the salt? — ordinary, among friends
- Could you pass the salt? — a shade more careful
- Could I ask you something? — asking for permission, politely
Nothing about the past is involved here. Could is simply at a lower volume, which is why it dominates emails and requests to strangers.
Could as possibility
Here could means “it is possible”, and comes close to might:
- It could be the postman.
- This could go wrong.
Can cannot replace it. It can be the postman means something else entirely — that such a thing is possible in general, not that it may be happening now.
Can’t and couldn’t
- Can’t is the flat impossibility: That can’t be right.
- Couldn’t is its past: I couldn’t sleep.
Can’t also works as a deduction about now — she can’t be home yet, she only left at six — which is the mirror of must be: see the page on must or should.
Could have
Could have + past participle covers what was possible but did not happen:
- You could have told me. — you didn’t
- We could have missed the train. — we nearly did
In speech it contracts to could’ve, which sounds exactly like could of. Could of is never correct in writing.
Mistakes to avoid
Wrong: I could to swim when I was six. correct: I could swim when I was six.
Modal verbs take the plain verb with no to: can go, could go, must go.
Wrong: He can could help. correct: He could help.
Two modals cannot stand together. Pick one.
Wrong: Yesterday I could find my keys at last. correct: Yesterday I managed to find my keys at last.
For one thing that worked on one occasion, English uses managed to or was able to. Could covers general ability, not a single success.
Wrong: She cans play the piano. correct: She can play the piano.
Modals never take -s, whatever the subject.