Wouldn’t you love to know why The Nine-Year-Old asked this question? Yeah, me too.
Sadly, this question came up during my annual August vacation, and while I dutifully wrote it down to answer later, I failed to include any information on its context. (At the time I probably thought I was doing well to write the question down at all. Not that that helps me in any way this morning.)
Let’s just assume that it was something The Nine-Year-Old read.
So, what’s the difference between thee, thou, and thy?
Thee, thou, and thy are all archaic singular forms of the second person pronoun you, which is pretty much why we only hear them in religious contexts these days. According to Quora, thou is used as the subject of a sentence, thee as an object, and thy to indicate the possessive. Wrapping all that into a sentence or two:
“Would thou like a pastry with thy tea? I would be happy to bring it to thee.”
So where does thine fit in?
Thine is an alternative version of the possessive form, typically used before words that begin with a vowel. You would use thy if the possessed noun begins with a consonant. In either case, you would use thy or thine when addressing only one person.
“Thy grasp of grammar is remarkable, if slightly old-fashioned.”
“Thine octopus ate my dictionary.”
What if I need to talk to more than one person?
Thee, thou, thy, and thine are all used when addressing only one person. If you need to talk to a crowd, you would use the plural form, ye.
“Gather ye octopus while ye may, for octopus meat is tender and yummy when served with chorizo and potatoes.”