Couldst is an old way of saying “you could”, but only when you were talking to one person. In that older English the word for “you” (one person) was thou, so people said thou couldst. “Thou couldst not see it” means “You could not see it.” Nobody uses this in ordinary talk now — not at school, not in email, not in shops. You meet it in the Bible, in Shakespeare, in hymns, and in fantasy stories or jokes that try to sound old. Though actually, the little ending -st is doing a simple job: it marks the verb for thou. You see the same piece in canst, wouldst, and shouldst. If you need this meaning today, say you could.

古英語「你能夠／你本可以」（thou 專用）