This is the 4824th most frequent Korean word.
공이
"공이" means "ball" in English.
In this sentence, '공이' refers to the subject of the sentence, which means 'the ball' and is the nominative case.
공이 둥근 모양으로 굴러갔다.
The ball rolled in a round shape.
Here, '공이' is used as the subject of the sentence, where '공' refers to 'ball' and '이' marks it as the subject.
그는 공이 너무 작다고 말했다.
He said the ball is too small.
In this context, '공이' specifies that 'the ball' is the doer of the action described in the sentence.