English possessives ("my," "his," "her") never change form. French possessive adjectives do, they agree in gender and number with the noun they introduce, not with the person who owns it.
1. Agreement: with the thing owned, not the owner
| mon livre | my book (masc. sing.) |
| ma maison | my house (fem. sing.) |
| mes livres / maisons | my books / houses (plural) |
2. The full table
| Owner | Masc. sing. | Fem. sing. | Plural |
|---|
| my | mon | ma | mes |
| your (tu) | ton | ta | tes |
| his / her / its | son | sa | ses |
| our | notre | notre | nos |
| your (vous) | votre | votre | vos |
| their | leur | leur | leurs |
3. Watch out: son/sa/ses don't reveal the owner's gender
Common mistake: unlike English "his" vs. "her," French son/sa/ses only agrees with the noun that follows, it says nothing about whether the owner is male or female. son frère can mean "his brother" or "her brother." You need context (or the person's name) to know which.
4. Before a vowel sound: ma/ta/sa become mon/ton/son
Just like with le/la becoming l', a feminine noun starting with a vowel sound (or a mute h) forces ma, ta, and sa to switch to the masculine-looking mon, ton, son, purely for pronunciation. The noun is still feminine.
| mon amie | my (female) friend (not "ma amie") |
| ton école | your school |
| son histoire | his/her story |
5. Quick reference
| mon / ma / mes | my |
| ton / ta / tes | your (tu) |
| son / sa / ses | his / her / its |
| notre / nos | our |
| votre / vos | your (vous) |
| leur / leurs | their |