"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.

Subjective Objective Possessive Present Tense

Verb Ending 1st Pers. Sing. I me my/mine[1] none 2nd Pers. Sing. thou thee thy/thine[1] -est 3rd Pers. Sing. he/she/it him/her/it his/her/its -eth 1st Pers. Plural we us our none 2nd Pers. Plural ye/you[2] you your none 3rd Pers. Plural they them their none [1]: "Mine" and "thine" were used before "h" and vowels, much as "an" was. [2]: "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 I am I have thou art thou hast he/she/it is he/she/it hath we are we have ye are ye have they are they have

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.