Regarding thou - my fields are medieval English and the history of English, and Oskar is quite right. Before the Norman French invaded England in 1066, thou/thee was simply singular, ye/you was plural. I don't think royalty used the plural, but I could be wrong. In the 13th century, ye/you still usually meant plural, but people started using it to show respect. They borrowed the idea from the French distinction between tu and vous - French tu used to be singular for neutral politeness, and vous neutral plural or respectful singular. The idea is something like, you're more important than just one person, so I'll address you in the plural.

By the time Shakespeare wrote his plays, thou/thee tend towards intimacy/informality, and ye/you tend towards respect/formality. Informal thou/thee was starting to sound insulting in certain contexts. If you look at scene 1 of King Lear, you'll notice that Lear addresses Cordelia formally with ye/you until he gets upset - they're both royalty and it's a formal occasion. When she upsets him, he switches to thou/thee, which is inappropriate for royalty in public, though it would have indicated affection in private. (Lear also uses thou/thee in scene 1 when he's talking legalese to the other two daughters.) This intimate/formal distinction comes up in John Donne's poetry in the 17th century. In Uncle Tom's Cabin, the Quakers are using thou/thee for intimacy, but by that time nobody else was doing that.

People got more polite to each other, to put it facetiously. You and vous started out as respectful forms and are now neutral forms in French and English. Thou/thee started dying out in educated speech right after Shakespeare. The Sanskrit professor here requires translations with thou/thee, but Oskar, I'm afraid that the Latin, Old Norse and Old English teachers here let thou/thee die some time ago ... do you want us to use it in our homework?

By the way, in case anyone is interested at all, Old English used to have a dual, but it had pretty much died out by around 1000. Some paradigms:

1st person. Sing.: ich (I), mec (me). Dual: wit (we two), unc (us two). Pl.: we (we), us (us).
2nd person. Sing.: thu, thech. Dual: yit, inch. Pl.: ye, eow.

(The spellings would have looked quite different in Old English, but this is how they were pronounced. My computer is refusing to cooperate with Eudora on special characters. My attempts to produce thorn come out as Þ and þ !)

Hope I haven't been wasting everybody's time,
E-Ching