Possessive adjectives (mon / ma / mes)
Summary
Possessive adjectives (mon, ma, mes, etc.) show ownership and agree with the thing possessed, not with the owner.
Explanation
Possessive adjectives replace the article and show who something belongs to. For one possessor the singular forms are mon / ma / mes (my), ton / ta / tes (your, informal) and son / sa / ses (his/her/its). For several possessors they are notre / nos (our), votre / vos (your, formal or plural) and leur / leurs (their). Each set has a masculine, a feminine and a plural form.
The decisive rule is that a French possessive agrees with the noun it precedes — the thing possessed — and not with the owner, as English "his/her" does. So mon frère, ma sœur, mes parents all belong to the same speaker; the form changes only because frère is masculine, sœur feminine and parents plural. In the plural (mes, tes, ses, nos, vos, leurs) there is a single form for both genders.
A pronunciation rule overrides the feminine before a vowel. Before a feminine singular noun beginning with a vowel or silent h, ma / ta / sa become mon / ton / son: mon amie (not ma amie), ton école, son histoire. The noun is still feminine; only the possessive form changes to avoid two vowels in a row.
Two pitfalls dominate. First, choosing the form from the owner's gender: son / sa / ses each mean both "his" and "her", so sa voiture is his or her car depending on context, and you must still pick son for a masculine noun even with a female owner (Marie aime son mari). Second, forgetting the mon/ton/son switch before a vowel. Rule of thumb: look at the following noun's gender and number to choose the form, swap ma/ta/sa to mon/ton/son before a vowel sound, and never let the owner decide the ending.
Examples
mon frère, ma sœur, mes parents — my brother, my sister, my parents (Same owner; form follows the noun's gender/number.)
mon amie — my (female) friend (ma becomes mon before a vowel sound.)
Marie aime son mari. — Marie loves her husband. (son (not sa): mari is masculine, regardless of the owner.)
C'est sa voiture. — It's his/her car. (son/sa/ses means both 'his' and 'her'.)
Les enfants rangent leurs jouets. — The children tidy up their toys. (leurs: several owners, plural noun.)
Common mistakes
Marie aime sa mari.
Marie aime son mari.
In English "his/her" agrees with the owner, so learners pick sa for a woman like Marie. But French possessives agree with the thing possessed, not the owner: mari is masculine, so it takes son regardless of who owns it — « Marie aime son mari », « Paul aime son mari ». Likewise son / sa / ses each mean both "his" and "her". Always look at the noun that follows, not at the person who owns it.
Prerequisites
Related rules
possessivesdeterminersagreement