This is the 81st most frequent Korean word.
건
The Korean word "건" can mean "thing," "matter," or "case," often used to refer to an object, event, or issue. It is also a contracted form of "것은" (geoseun).
'건' (a contraction of '것은') emphasizes the subject that 'this book being interesting' is the truth.
이 책이 참 흥미로운 건 사실이에요.
It is true that this book is indeed interesting.
'건' here refers to 'that thing' or 'that fact,' where it is a contraction of '그것은'.
그건 이미 알고 있어요.
I already know that.
'건' (contraction of '것은') refers to 'the thing that was given'.