"이것은 건물이 아닙니다."
Translation:This is not a building.
That doesn't really make sense in English. Its literally saying 'This building is not'
이것은 is used to refer to an object specifically as it has the word '것' which means 'thing' 이 can be used, but only when the object (or whatever is being talked about) is specified after like you wrote '이 건물' 이것은 is just used for saying 'this thing' as the object you are talking about can clearly be seen, so you don't have to say its name.
이 = The object being spoken about is closer to the speaker than the listener, e.g 이거 얼마에요? (How much is this?)
그= The object being spoken about is closer to the listener than the speaker e.g 그것은 신발 입니까? (Are those shoes?)
저 = The object is far from the speaker and the listener like saying "that thing over there" e.g 저것은 뭐예요? (What is that thing over there?)