Both mean "your" in older English. Thy sits before a word beginning with a consonant: "thy kingdom". Thine is used before a vowel sound — "thine eyes", "thine own heart" — and also stands alone where modern English would say "yours": "the victory is thine". It is the same alternation as a and an.
The core difference
These are not two ways of saying the same thing in different moods. The choice is decided by the sound that follows, exactly as with a and an.
The rule
- thy — before a consonant sound: thy sword, thy father, thy kingdom come
- thine — before a vowel sound: thine eyes, thine enemy, thine own heart
- thine — standing alone, where modern English uses yours: The fault is thine.
Where writers go wrong
Reaching for an archaic register, people scatter thine for effect: "thine sword" reads as wrong to anyone who knows the pattern, in the same way "an sword" would. If you would say an before the noun, use thine.
The wider set
Thou is the subject (thou art), thee the object (I saw thee), thy and thine the possessives. See thee vs thy for the object-versus-possessive half of the same system.
Frequently asked questions
Is it thy eyes or thine eyes?
Thine eyes. Eyes begins with a vowel sound, and thine is the form used before vowels — the same alternation as a and an.
Can thine be used on its own?
Yes. Standing alone it means yours: "the victory is thine". Thy cannot be used that way; it must sit in front of a noun.
What is the difference between thy and thee?
Thy is possessive and goes before a noun (thy sword). Thee is an object pronoun meaning you (I give this to thee).