used as a modal auxiliary in verbal phrases with present or future time reference, generally equivalent to can in meaning and use, with the following functions:
expressing esp. a shade of doubt or a smaller degree of ability or possibility: it could be so
expressing permission: could I go?
forming the present conditional: it would help if he could wait
forming the past conditional: he would have left if he could
expressing or suggesting politely less certainty than can: could you wait?