Question

Why is it called a "building" when it is already built?

Answer 1

its called a building becuz the ending of the building there is a ing. it is added to verb to make it a noun form
hope it helps :)

Answer 2

It becomes building as the suffix 'ing' is added to other words to make them nouns and in this case 'ing' is added to the word build making it a noun 'building'.

hope it helps..