I am new to German so would would be grateful if someone could help with this trivial question.
When doing some exercises from my course book I saw such example
Der Computer ist nicht neu. Es ist ein alter Computer.
The question is why in the second sentence they use "es ist" instead of "er ist".
Also there were some examples for feminine and neutral nouns as well (die Uhr, das Bild) and all of them used "es ist" in the second sentence.