could

Could is a modal auxiliary verb in English used to express ability, possibility, or conditionality. It frequently appears in past, present, and hypothetical contexts, and it can soften requests or suggestions.

Common uses include past ability (I could swim when I was younger), general or future possibility (It could rain tomorrow), and conditional mood (If I studied, I could pass the exam). It also appears in polite requests or offers (Could you help me? Could I borrow your umbrella?) and in suggestions (You could try restarting the computer).

In contrast to can, which denotes present ability, could often signals conditionality, hypothetical situations, or a more tentative meaning. It is also used to describe a past capability in a general sense, where was able to would describe a specific successful instance (I was able to finish the task yesterday after a long effort). The form could have expresses past possibility or unrealized events (It could have rained, but it stayed dry).

Could appears in indirect or reported speech as a past-tense reporting of ability or possibility (She said she could come). It is also used to discuss hypothetical or counterfactual situations in conditionals and narratives. When expressing permission or requests in a more courteous tone, could is common and often preferred to can or may in informal modern usage, while may remains more formal in some contexts.

Etymology traces could back to the past tense form of can, serving as a versatile modal that expands the expressive range of English about ability, possibility, and conditionality.