Possessive adjectives in French - called les adjectifs possessifs - are words used to indicate possession of a thing or person, or to make it clear that a thing or person belongs to a subject.
They follow the same rule as articles, since they agree in gender and number with the noun they refer to and are always placed before the noun , never after, as happens in Italian in some cases (especially due to different regional dialects).
Furthermore, remember that the French possessive adjective is never accompanied by the article , unlike in Italian. We would say "il mio amico," specifying that it is that specific friend with the definite article: this doesn't happen in French, and you can tell that "mon amie" is that specific friend from the context in which the sentence is placed and from the other elements that compose it.
Ton amie Marcus est très sympa. = Your friend Marcus is very nice.
In the plural form (we, you, they) there is no difference between masculine and feminine . Here too, therefore, you can only determine whether one or the other case is involved from the context.
In the sentence:
Je n'aime pas ses amis. = I don't like his /her friends.
We still can't figure out whether they are male friends or female friends.
If we add, however, the following sentence:
Ces deux-là, Jean et François, sont vraiment méchants. = Those two over there, Jean and François, are really bad.
so at this point we know they are males.
A very common mistake concerns the third person: many people use it as the subject who owns the thing, but in reality, the third person singular agrees in gender and number with the object referred to by the French possessive adjective. And in the singular, we distinguish between "suo di lui" and "suo di lei." It may seem obvious, but in reality, many French students make this mistake.
Tes lunettes de soleil = Your sunglasses.
Another special case concerns demonstrative adjectives. In Italian, it's not a problem to find possessive and demonstrative adjectives in the same sentence. However, when this happens in French, we have to make a choice: we can't write both the demonstrative and the possessive adjective in French. Most of the time, in these cases, the demonstrative is omitted because the possessive allows the sentence to be more or less preserved.
This friend of yours is nice = Ton amie est sympathique.
These cookies are delicious. Ses cookies sont délicieux.