Grammar
-거든(요)
give background explanation
Form
Verb or adjective stem + -거든(요): adds a reason or background explanation the listener may not already know.
Function
Introduces a reason or explanatory information the listener may not know; the nuance depends on its position and intonation.
Why this form?
-(으)니까 — “since, because”: directly connects a reason to its consequence; sentence-final -거든(요) supplies background explanation.
How to build it
Sentence-final -거든(요) supplies an explanation. Do not confuse it with conditional -거든 followed by another clause.
Forms not to confuse
-(으)니까
피곤하니까 쉬세요. — Since you are tired, get some rest.
Examples
너무 귀찮거든. — Because it’s too much trouble.
오늘은 집에 있을게요. 피곤하거든요. — I'll stay home today. I'm tired, you see.