1) -거든요 is usually added separately about what happened or will happen. 저도 모르겠어요. 저 방금 왔든요. I dont know. i just arrived here. 2) when your sentences end with -거든요, the other person will expect that you will another thing related to the first sentence. 저 지금 돈이 없거든요. 만원만 빌려 주세요. ---- 때문에 is used to express "because" 1) 자존심 탓 때문에 말도 못 해 이렇게 ----> because of my pride, I cant say it. (From Girls Day Expectation song hahahaaha)

-거든요 is a verb ending that goes at the end of a sentence, and roughly means, "you see?". For example, in English: "I couldn't go to the library today. It was closed, you see?" That is certainly similar to "I couldn't go to the library because it was closed". -거든요 is used as kind of a followup explanation. 때문 is a noun, which means "reason", that can be used in a setup clause like "Because the library was closed...", in which case you add the particle 에: 때문에.... It can also be used at the end of a sentence to mean "It's because...". In that case, you use the verb 이다 with it: 때문이다. I'm not a native, so hopefully someone will give you a more complete explanation.