"Canst" is a direct address to the second person singular. You must use it with "thou". Sound odd? Yes, it's old English. If you use it, you would sound like someone from a Shakespeare play. For modern speech, use "can" for all occasions. :)

"Canst" is not a word in the modern English language. It is from Shakespeare's time: if you try to use it today, no one will know what you're talking about.