French possessive adjectives are going to follow similar rules than articles because both are affected by gender and number. Getting them right is therefore going to depend on knowing if the noun accompanying them is feminine, masculine or plural.
‘My’ is translated by ‘mon’ if the following word is masculine (m),by ‘ma’ if the following word is feminine (f), by ‘mes’ if the following word is plural (pl).
‘Your’ is translated by ‘ton’ if the following word is masculine (m), by ‘ta’ if the following word is feminine (f), by ‘tes’ if the following word is plural (pl).
mon sac (m) noir – my black bag
ma montre (f) en or – my gold watch
mes lunettes (pl) de soleil – my sunglasses
ton sac (m) noir – your black bag
ta montre (f) en or – your gold watch
tes lunettes (pl) de soleil – your sunglasses
The translation for ‘his’ is going to follow the same pattern as for ‘my’ and ‘your’, i.e. it is going to be dependent on gender or number. The only confusing issue for the English learner is that the French does not indicate if the person possessing the object is a woman or a man, i.e. there is no difference between ‘his’ and ‘her’. The possessive adjective agrees with the noun that follows not the person that owns it.
son sac (m) noir – his / her black bag
sa montre (f) en or – his / her gold watch
ses lunettes (pl) de solei – lhis / her sunglasses
Note. Only the context can give you a clue about the owner of the object or person.
Elle est venue avec son mari et sa fille – She came with her husband and her daughter.
Il n’a pas beaucoup de chance. Sa voiture est en panne. Son pneu est crevé – He’s not very lucky. His car has broken down. His tyre is flat.
Another difference with English is the fact that ‘your’ has 2 other forms corresponding to the singular (sing) formal speech and the plural (pl) but is not affected by gender. ‘Our’ is translated exactly in the same way.
Est-ce que votre fille s’entend bien avec votre fils? – Does your daughter get on with your son?
Est-ce que vos enfants aiment votre chien? – Do your children (pl) like your dog (sing)?
Notre maison (sing)est assez grande pour tous nos invités (pl) – Our house is big enough for all our guests.
In French, ‘their’ is affected by plural but not gender. In the singular form it is translated by ‘leur’ and by ‘leurs’ in the plural.
Leur grand-mère (sing) est décédée hier soir – Their grand-mother has died last night.
Leurs parents (pl) étaient très attristés – Their parents were very sadened.