personal pronouns

how do you know to use or to translate "la sienne or le sien"?  His or Her and for that matter the same question about "son"?
For example "Ce n'est pas son jouet. C'est le sien"  a boy and a girl fighting over a toy - how do you know if "son" is her or his and the same for "le sien" - is it his or her?

You would think that "la sienne" would be for a femine speaker and the "le sien" would be for a masculine speaker - but this is not always the case - why?

The personal pronouns match the thing that's owned, not the (gender of the) person who owns it. So because jouet is masculine (le jouet), it's always son/sien/mon/mien/ton/tien, irrespective of who owns it. So yes, in that example, it doesn't "resolve" who actually owns it. You would need to point to the owner or something else.

The exception is when the possession begins with a vowel, then the mon/son/ton is retained to avoid a two vowel sound. This is why we say mon amie (female friend), mon école, mon adresse, even though they are all feminine nouns.

Thank you for the quick response - so if you are reading this in a book or letter - you would need to organize your sentence some other way to make clear who owns the object?

One more question - how would you write "this is his car" or "this is her car" since you can not point to a person?

Tks

Exactly, you would need some other context, like « Anne est allée nous voir. C'est sa voiture dans le garage. » {Anne came to see us. It's her car in the garage.»} For "This is his/her car", you would use the same phrase: C'est sa voiture. {or « Cela, c'est sa voiture. »}.

Thanks so much - this clears it up for me