Thou; thee and thine: how we used to speak English.

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Early Modern English is the stage of the English language from the beginning of the Tudor period to the English Interregnum and Restoration in the late 15th century, to the transition to Modern English, in the mid-to-late 17th century.

The grammatical and orthographical conventions of literary English in the late 16th century and the 17th century are still very influential on modern Standard English. Most modern readers of English can understand texts written in the late phase of Early Modern English, such as the King James Bible and the works of William Shakespeare, and they have greatly influenced Modern English.

Early Modern English used words like thou (you 2nd pers. sing. Subj.); thee (2nd pers. sing. Obj.) and thine (2nd pers. sing. Poss.). These words originate from Proto-German, the common ancestor for all Germanic languages. So thou means Du; thee as the same as Dir and thine would be Dein.

Thou is pronounced the same as Du, execpt with the ð sound at the beginning (which, by the way, is an inheritance from the Vikings).

Here is a paper from the University of Cincinatti about this topic.

May I also suggest having a look at the videos from Simon Roper and Rob Watts on YouTube.