거 든요 : 어떻게 사용하는 건지 설명해주세요?
Hello i am currently learning the form "-거 든요" I know this can have different meanings. Right now the only time i use it is when i am responding to a question.
왜 학교에 안 왔어요?
너무 피곤해서 못 일어났거든요

Could you please let me know the other meanings? And most important, give me exemples so i can have an idea of how to build the sentences.

Many thanks

Question

거 든요 : 어떻게 사용하는 건지 설명해주세요?
Hello i am currently learning the form "-거 든요" I know this can have different meanings. Right now the only time i use it is when i am responding to a question. 
왜 학교에 안 왔어요? 
너무 피곤해서 못 일어났거든요

Could you please let me know the other meanings? And most important, give me exemples so i can have an idea of how to build the sentences.

Many thanks

Answer

We often use this " ~거든요" for describing some reasons which caused something to happen in the past or still affecting current situation now .
like you said, when someone asks questions, we use this pattern to answer.
For example,
1) 요즘 왜 이렇게 살이 쪘어요?
    제가 요새 스트레스 때문에 밤에 간식을 많이 먹거든요.

2) 태양의 후예는 제가 좋아하는  드라마에요. 제가 송중기 팬이 거든요.

3) 저는 지하철을 자주 이용해요. 그게 제일 시간을 맞출수 있는 방법이거든요.
 
Hopefully it helps you understand this pattern..