"The cat is black; it is a black cat."
Translation:Die Katze ist schwarz; es ist eine schwarze Katze.
This is a good question which makes me think. I would say it is ok to use "sie" hier, even though "es" is somewhat more common. Let me list a few sentences the way I would say them.
"Die Katze ist schwarz; sie spielt mit dem Ball." "Die Katze ist weg; sie sitzt auf dem Baum." "Ich finde die Katze nicht; sie ist weg."
In none of these cases you can replace "sie" with "es". It would always be wrong. Maybe someone who understands grammar better than me can explain what is going on.
I am no Englishman and no German. But in my opinion the explanation could we find in these english senteces: This man is tall; he is a tall man., or: This man (masculine) is tall; it (neuter) is a tall man. If the last sentence is possible, the the german sentence "Die Katze (feminine) ist schwarz; es (neuter) ist eine schwarze Katze." is correct.
I think it has something to do with "Katze" here as being said without knowledge of its gender, for example a "Kater" is a male cat, and "Katze" could be either a "female cat" or refer to the whole race in general. I think in that sense, both "es" and "sie" would be accepted. Just a guess.
My German friend says:
Sie ist eine schwarze Katze. -> My friend has a pet named Mauzi. "She's a black cat." -> The cat is a mammal. She is often a pet.
Es ist eine schwarze Katze. -> Something moves in the bushes. It's a black cat. -> What animal do you see in the photo? It's a cat.
That's what I think is the main difference. One is refering specifically, the other more pointing.
It may be something like: A: "There's some one at the door." B: "Who is it?" A: "It's a tall man." You wouldn't say: "He's..." in this case because you're interest is the man/the person at the door.
A: "What's your teacher like.?" B: "He's a tall man." Here we're discribing a man whom we know.