"Thou", "thee", "thine" and "thy" are pronouns that have dropped out of the
main dialects of Modern English. During the period of Early Modern English (~1470-1700),
they formed the Second Person Singular of the language, and were standardized
by the time of the King James Bible as shown below.
1st Pers. Sing.
2nd Pers. Sing.
3rd Pers. Sing.
1st Pers. Plural
2nd Pers. Plural
3rd Pers. Plural
: "Mine" and "thine" were used before "h" and vowels, much as "an"
: "You" had replaced "ye" for most plural uses by 1600.
Here are the conjugations from that era of two common irregular verbs:
to be - Present tense
to have - Present tense
You may have been told that "thou" and "thee" were for familiar use, and "you" and "ye" were formal. This was not true originally, but it was true for about two centuries, roughly 1450-1650, including Shakespeare's time. The previously plural "you" was used in the singular to signify politeness and respect, which left "thou" and "thee" for all the other singular uses, ranging from endearing intimacy to bitter rudeness. Eventually, the politer "you" drove out nearly all uses of "thee" and "thou"; they survived mostly in poetry and religion.
Several groups continue to use these pronouns today as part of their
daily speech (although with different grammar), including residents of
Yorkshire, Cumbria, the East Midlands, and some rural areas of Western
England. Some Quakers also used their Plain Speech with "thee" and "thy"
until the middle of the 20th century.