How and when to use "거야" in Korean?

Question

i see that people often use "거야" in Korean. Such as

집을 좀 알아봤는데 방이 다 손바닥만 한 거야

i think the sentence can write as "집을 좀 알아봤는데 방이 다 손바닥만 해". That's enough. So why we need to write "집을 좀 알아봤는데 방이 다 손바닥만 한 거야"? and How and when to use "거야" in Korean?

Answer 1

손바닥만한해 vs 손바닥 만한 거야

(1) The first is about fact and the second is about experience or non-experience.

Assume that someone did not go to north pole and south pole. But through a book we know that "north pole is cold as much as south pole (북극은 남극만큼 추워)"

Assume that someone, who lives in south pole and believe that there is no place like south pole, went to north pole recently. Then he says that "north pole is cold as much as south pole and I am bewildered (북극은 남극만큼 추운거야. 나 당황했어)"

(2) 표현 :

(2.1) When a baby throw an apple, then mother says that "an apple is eatable (사과는 먹는거야)".

(2.2) 사과는 먹는 거고 연필은 먹지 못하는 것이다. An apple is eatable thing and pencil is not.

(2.3) 자 둘걸 I should have slept.

(2.4) It is true that law is useful and sometimes it is not for someone. When someone lose the trial, he say that law is useless. Then we can ask "really (그런거야 =You reached the conclusion through your experience) ?"

(2.5) 거야 is usually not used to the older.