or archaic 2d sing couldst
Etymology: alteration (influenced by should and would ) of Middle English coude, couthe, from Old English cūthe; akin to Old High German konda could, Old Norse kunna, Gothic kuntha — more at can
past of can — used in auxiliary function in the past tense
he found he could go
in the past conditional
he said he would go if he could
and as an alternative to can suggesting less force or certainty or as a polite form in the present
could you do this for me
and the present conditional
if you could come we would be pleased