English often uses 'their' as gender-neutral third person singular, could 'their dog is black' not also be acceptable?
"son" is not at all gender-neutral third person. It really means his or her.
Right I understand that, but I'm talking about the English translation, not the French grammar. I just don't see why 'their dog' is not acceptable as an English translation, particularly when we have no other context to work off as to whether it is his or hers.
I understand as well, but in the absence of context, you cannot speculate on the fact that the single owner suggested by the French sentence would become several owners in English.
If some English people use 'their' as a third person singular, they are wrong.
Actually it isn't wrong, English isn't a regulated language like French is and it is common and acceptable for 'they' to be used as a non-gender-specific third person singular. But this English grammar discussion is beside the point :)
Yes, technically speaking, using "their" for gender-neutral third person is grammatically incorrect. Instead, you should actually say "his or her" which refers to a singular possessive, whereas "their" refers to a plural possessive. Some people find it easier to just say "their" instead of his or her and the meaning is conveyed fine, but just for the record it's not correct.
Incorrect: A person (singular) should raise their (plural) hand to speak.
Correct: A person (singular) should raise his (singular) or her (singular) hand to speak.
Correct: People (plural) should raise their (plural) hands to speak.
Yes, since "son" is not specific to the owner but to the gender and number of the object owned.
what's the correct one? they said it was "her dog". but here it's "his dog". I don't understand