Today, in honour of the Shakespeare 400-year anniversary, a quick look at thee and thou. I see people mix these up all the time and all it really comes down to is a lack of understanding of grammar.
Thou is nominative (subject form), thee is object form, and thy/thine is possessive form.
The use of thy/thine depends on the object of which you are speaking. A good general rule is thine goes before words beginning with a h or a vowel. This same holds true if you are choosing between my and mine.
Here are some examples:
Thou art a fool.
All my foolishness I give to thee.
Thy foolishness offends me.
Thine eyes are full of mischief.