Thou takes the subjective form, thee is objective.
When the next word begins with a vowel, use thine for thy.
I like thy face, but
I applaud thine effort.
The "-eth" ending is only used with he, she, and it.
He loveth best that loveth well.
God knoweth why!
Using Thou familiarly:
Thou and thee are familiar or informal forms of you. It is used to address your children, your servants, your wife, your most intimate friends, your dog, and God. (Hey, who knows you better than God?)
Use the more formal you when addressing your parents, your master, your social superiors, your patron, your customers, your officers, and your horse, who may be worth as much as you are.
I recently caught an old John Wayne movie called Angel and the Bad Man in which the heroine, a Quaker, took liberties by addressing John Wayne's character as thee and thou. Well, what do ya know about that? I wondered at the revelation this usage suggested. So having the worldwide web at my fingertips, I googled it and found the grammar lesson I copy-pasted above.
Did you see what I saw? Isn't that neat?
My whole life I've prayed my prayers the way I learned them at my Mother's knee, using Thee and Thy and Thou. It's always just sounded right to me because it's what I'm used to, and I guess I just thought it sounded more respectful. Or something. But, what a wonderful thing to learn, especially at this stage of my life, that using the familiar terms Thee and Thou when I'm talking to Our Lord is really a symbol of love and closeness. They are terms of endearment.
Saying "My God, I love Thee" says it twice.
Here is our Creator, far more than just a friend, infinitely above us, but more dear, more intimate to the very soul of us than any of our earthly loved ones. His image is on the beautiful crucifix above the altar, and there He is, truly present, raised above our priest's head at the Consecration. He looks down on me and I look up at Him with new understanding -- and I can just say Thou.