Ce sont les siennes
I would have expected it to be: Ces sont les siennes. Could somebody explain why the singular is used?
Certainly. This can be a little confusing since there are two parts of grammar involved here.
C'/Ce + the verb être are demonstrative pronouns. C'est = it's / Ce sont = they are.
Ce, cet, cette, and ces + a noun are demonstrative adjectives. Ce garçon = this or that boy / ces filles = these or those girls / cet appartement = this or that apartment.
Now if you change the sentence a bit and put a plural noun after "ce" you can have: Ces robes sont les siennes.
If I'm not mistaken, that sentence means "these are his," correct?
If so, that why is it "les?" I thought that "his" in this context, was "le." Do you also make "him" plural?
Maybe I've got it all mixed up! Je ne sais pas!
It's not only that "These are his (or hers)" but more specifically "These (grammatically feminine objects) are his (or hers)." It's « les siennes » because the objects they refer to are plural and feminine. If it were one (grammatically feminine) object, it would be « C'est la sienne » (« C'est » is a contraction of « Ce est »). If it were multiple (grammatically masculine or mixed) objects, it would be « Ce sont les siens ».
In English, possessive pronouns do not agree with number (or, obviously, gender) of what they replace, but in French, they do. The Possessifs-2 skill's tips has the rundown.
Now, why must you use « C'est » and « Ce sont » and not « Il est, elle est » or « Ils sont, elles sont » with these possessive pronouns? Just because that's how French is.