"This is not a building."
Translation:이것은 건물이 아닙니다.
It's just correct to say "this thing" in Korean vs. "this." As for 이것은 vs 이것, the former is grammatically correct, and the latter is more spoken style, leaving off particles because one is lazy/speaking fast.
I believe it is not just 이 because 이 modifies whatever "this" thing is. If it were affirmative, as in "this building [did something]", it would be "이 건물은", but because it is saying that "this [thing] is not a building [but rather a chicken]", it should say "이것은 ...." The ending 은 is used to signify the subject of the sentence is "This" and that the following clause describes what it is (or isn't). People please correct me if I'm wrong!
i might be wrong, but it could have something to do with the form of this, like a general article but yeah idk for sure
Wait! I think I've noticed something.
The reason why 건물 has the subject suffix 이, even though it is not really the subject(in my opinion) of the sentence is because the verb 아닙니다 is a copula verb. The verb connects both 그것 and 건물, making it impossible for the latter to receive the object suffix.
Am I correct in this?
Yup, good job. You're right, in English, building is not the subject. "This" is. However Korean has topic particles, which English doesn't have, thus this is a good example of seeing them both in action.
In addition to functioning as a subject particle, 이/가 can be a complement particle for copula verbs (in this case 아니다). In English grammar, 건물 would be considered a subject complement with 아니다 as a linking verb.