Thou, thee, etc., are old forms which are now out of use in ordinary speech. Usage was based on position:
Subject pronoun: thou (singular), ye (plural) <you>
Object pronoun: thee (singular & plural) <you>
Possessive pronoun: thine, thy <your, yours>
There are, however, two modern uses of thou, thy, etc.:—
(1) In elevated style, especially in poetry.
2) In addressing the Deity, as in prayers.
Read more here English Language : PERSONAL PRONOUNS