This is the 26th most frequent Korean word.
것이
"것이" means "a thing that is" or "what is."
In this sentence, "것이" refers to "this thing" or "this object", functioning as a noun phrase indicating a specific item.
이것이 무엇인가요?
What is this?
In this sentence, "것이" represents the specific issue or thing that causes the problem, acting as a representation for an abstract concept.
문제가 되는 것이 무엇인지 확인하세요.
Check what is causing the problem.
Here, "것이" serves as a nominalization of the verb phrase "공부를 열심히 하다" (to study hard), converting it into a subject of the sentence.