-군 is an ending to imply the speaker gives attention and mild admiration to something s/he has just known. And -요 is a honorific ending. So you can use just -군 when you talk to your close friend(your age or younger than you). When you talk to someone older than you, you should use the two, -군 and -요. A : 왜 아무도 안 왔어? Why haven't they gotten here yet? B : 지금 차 막힌다던데. There's a traffic jam now. A : 미치겠군. It's driving me nuts. A : 남동생 몇 살이에요? How old is your brother? B : 7살이에요. He's 7 years old. A : 7살요? 내년에 학교 들어가겠군요. He would start school next year. "a verb excluding '다'(basic ending) + -는군요" is in present tense. For example, 잘 하는군요, 지금 가는군요. But -군 isn't very often used in present tense, in my observation. 거든 is an ending to imply the speak inform the listener about something the listener wouldn't know, with admiration or boast. A : 왜 기운이 없어요? Why are you so down? B : 감기 걸렸거든요. I've got a cold. Hope this helps.

basically -군요 and -는군요 are almost same, both of them are interjections. the only difference is how they are used. like the examples ooti gave, -군요 is next to the nouns, -는군요 is next to the verbs -거든요 is a description but it has a subtle difference to reconfirm. hope this helps you =)

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