My, your, his, her: mon, ma, ton, ta, son, sa
The agreement in French is between the adjective and the following word, but the rule is not as clear as expected.
1. Livre is masculine in French so we use Mon, Ton, Son
2. But we also use the adjectives with feminine words starting with a vowel: son école (school)
3. And also with words starting with a mute H: son huile (oil)
1. Voiture is feminine and starts with a consonant, so we use Ma, Ta, Sa
2. Also with words starting with an unplugged H: sa hache (ax, axe)