I have seen sentences like these that really confuses me such as:
"Das sind unsere."
"Das sind Katzen."
As far my as understanding allows the definite articles (der, die, das, die) can also be used as demonstrative pronouns this/that. If that's the case here why does the pronoun NOT agree with the subject? Doesn't "das" represent singular neuter while "unsere" and "katzen" are in plural?
As stated above "das" is the common demonstrative pronoun - independent of gender and number. You can thus say
Das sind meine Brüder.
And it would be the generic choice. But "der/die/das" is a demonstrative pronoun as well. So what you are seeing is technically not the definite article, but a pronoun that is identical to it! As a demonstrative pronoun alone "der/die/das" has 10 sub uses in my dictionary.
But I believe what you are interested in here is the contrasting use of the demonstrative "der".
I try to make an example where both "das" and "der" are used idiomatically, the first neutrally and the second for contrast:
Wer ist der Mann mit der Sonnenbrille?
Das ist mein Freund!
Und was ist mit Martin?
Der hat mich verlassen!
Translation:
Who is the man with the sun glasses?
That's my boyfriend!
And what about Martin?
He (contrasting) has left me!
More examples of "der/die/das" as a contrasting demonstrative pronoun include:
Welcher ist dein Vater? / Which one is your father?
Der mit der Glatze... / The one with the bald head.
The word "das" is normally used as "generic" demonstrative pronoun.
When the word does not refer to another word already mentioned you use "das":
These are our cats.
You use the word "das".
Only when the pronoun refers to another word already mentioned you can use "der" or "die":
Q: Wessen Schlüsselbund ist das?
A 1: Der ist mein Schlüsselbund.
A 2: Das ist mein Schlüsselbund.
A 3: Der gehört mir.
In the question the demonstrative pronoun does not refer to another word. So the word "das" is used.
In the answers (1) and (3) the pronoun refers to the word "Schlüsselbund" (in the question) so the word "der" is used.
In the answer (2) the pronoun "das" does not refer to the word "Schlüsselbund" already said.
Most Germans would use answers (2) and (3) but they would not use answer (1) for some reason I don't know.
