If you want convincing story-lines that include "old-fashioned" or fantasy-sounding dialogue with Thee and Thou, but you aren't sure how to make it convincing, this is for you.
It's easy if you take a little care and maybe practise a bit. The Book of Common Prayer is full of examples.
Thou works like You except it's singular:
I am going to the bar, because I am thirsty.
Thou goest unto the inn, for thou art thirsty!
You two are going to get a spanking.
He goeth so slowly he hath grown roots.
We go every day.
They go too.
Notice how the verb (the doing-word) changes too. thou gets >i>est</i> and he (or she or it) gets eth.
There's always an exception: are
I am / you three are / thou art / he is / we are / they are
The -eth ending happens now. He runneth, thou runnest (or run'st). Tomorrow, I will run, thou shalt run, he shall run, they'll run, and so on.
Never use -eth for the past or future tense. Use runneth just exactly when you would