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
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