Our language’s second-person pronoun used to be more complex than the simple you and was declined as follows: thou and ye as subjective singular and plural; thee and you as objective singular and plural. The plurals evolved into the preferred forms in most formal, and eventually informal, settings, and their pronunciations merged into what we use today.
From late Middle English to early Modern English, the singular forms—thou, thy or thine, and thee—were preferred only in informal, familiar, and condescending situations. Thou even doubled as an insulting verb meaning to refer to someone as inferior: “Thou viper, for I thou thee, thou traitor!” (Sir Edward Coke to Sir Walter Raleigh, during the latter’s trial, 1603). This historical nuance has been lost to modern speakers, who mistakenly believe that the archaic forms were formal or honorable, possibly due to their use in the King James Bible, whose writers may have wanted to portray god as familiar and close to man rather than an unapproachable creator.
Avoid the kitsch habit of using the archaic forms to emulate formal and aged speech. A favorite of fantasy writers, it is nonetheless uninformed and wrong.