Can vs. Could
Can and could overlap in ability, possibility and requests, but could is not simply a past-tense substitute in every use. Could can mark past general ability, weaker possibility, hypothetical situations, or a more tentative/polite request.
Use can for present ability and many direct present possibilities or permissions; use could for past general ability, hypothetical or weaker possibility, and often more tentative requests.
Key details
Choose the right form
| Meaning / situation | Construction | Decision |
|---|---|---|
| present ability or a direct present permission/possibility | can | usually the more direct form |
| general past ability, hypothetical/weaker possibility, or a more tentative request | could | use when the meaning calls for distance or tentativeness |
Quick tests
Time-and-distance test
Are you stating present ability or making a direct present request/permission statement?
If yes: “Can” is often the natural choice.
If no: Use “could” for general past ability or when you intentionally make possibility/request wording more tentative.
Caveat: A single successful past event may need a construction other than bare “could”; interpret the specific sentence rather than applying a one-line tense swap.
Examples
I can meet at three this afternoon.
“Can” expresses present ability/availability.
When I was younger, I could run five kilometres easily.
“Could” expresses general past ability.
Could you send the revised file today?
“Could” makes the request more tentative/polite than a direct “Can you…?”
Further guidance
Ability Boundary
Could commonly expresses general ability in the past; single successful past events often need a different construction.
Possibility Request Boundary
Could can make present/future possibility or requests more tentative than can.
Sources and evidence
Sources are shown with the role they play in this guide. Historical or style-sensitive claims are kept within the evidence boundary described above.
- Can, could or may? — Cambridge Grammar (opens in a new tab)Cambridge Dictionary · Can and could across ability, possibility, requests and permission