This is the 1954th most frequent French word.
siens
"Siens" means "his," "hers," or "its," referring to possessions or things belonging to someone.
Here, 'siens' is a possessive pronoun, indicating ownership and agreeing in gender and number with 'livres'.
Les livres sont les siens.
The books are his.
In this sentence, 'siens' again serves as a possessive pronoun, representing ownership of the pens 'stylos' and conforming to their masculine plural form.
J'ai trouvé les stylos, mais je ne suis pas sûr qu'ils soient les siens.
I found the pens, but I'm not sure they are his.
Here, 'siens' refers back to 'jouets'. It functions as a possessive pronoun to assert ownership by Marie and matches the masculine plural form of 'jouets'.