This is the 19401st most frequent Korean word.
애가
"애가" means "child" or "kid" in English.
In this sentence, "애가" refers to 'the child' subject, where "애" is a colloquial term for a child and "-가" is the subject marker.
애가 울고 있어요.
The child is crying.
Here, "애가" again refers to 'the child,' specifically "그 애" indicating 'that child,' as the subject of the action.
그 애가 뛰어갔어요.
That child ran away.
In this usage, "애가" means 'this child,' where "이 애" specifies the child as 'this one,' and "-가" marks it as the subject.