Es ist richtig, daß man normalerweise nicht sagt "Der ist ein Apfel" oder "Die ist eine Katze" (although it occurs to me that you might say that in colloquial German if a man were dressed as an apple or a woman as a cat :) ) Perhaps it's easiest if you remember that in answer to the question "Was ist das?" (always in singular, even if you're asking about something that's clearly in plural, because the plural is so awkward in German, you'd have to say "Was sind das für welche?") it's always "Das ist/sind..." Because if we've got an apple and a pear in front of us, I might point to them, saying (!) "Der ist sauer, die ist süß" (That one's sour, that one's sweet/That's a sour one, that's a sweet one), because we know that they're apples and pears, we know that it's "der Apfel", but "die Birne", and it's therefore impossible to say "Das ist sauer, das ist süß" instead. Equally, in plural you would be saying "Das sind Äpfel, das sind Birnen." (What are those? - Those are..), but if you were specifiying, you'd say "Die sind süß, die sind sauer". Please note that this is spoken German. In written German, it would be "Diese ist süß, dieser ist sauer", it's considered sloppy to use "der/die/das" as a demonstrative pronoun. It would be impossible to say "der sind", because the definite article in plural is always "die" :) Ich hoffe, das hilft?