Difference between these words?
여러분 / 여러분들 what does the 들 actually do in everyone when it's already plural?
Do you use "what" or "when" when asking time? 뭐? 언제? or *?
How do you call a person in his 40-50 but you're close with? is it 형님? 누나님? assuming that I'm 18
Difference between 몰라요 and 모르겠어요?
Thank you.

Question

Difference between these words?
여러분 / 여러분들 what does the 들 actually do in everyone when it's already plural?
Do you use "what" or "when" when asking time? 뭐? 언제? or *?
How do you call a person in his 40-50 but you're close with? is it 형님? 누나님?  assuming that I'm 18
Difference between 몰라요 and 모르겠어요?
Thank you.

Answer

1. 여러분 / 여러분들 : '~들' can be located after root of words (both singular and plural).
2. '언제' means 'at a specific time', so it has similar meaning to 'when'. But If you want to ask only exact time, (like a what time is it now?) correct sentence is '지금 몇 시(what time) 입니까?' 
3. How do you call a person in his 40-50 but you're close with? is it 형님? 누나님? assuming that I'm 18 => In this case, you can be called '아저씨'(masculine) and '아주머니'(feminine).
4. '몰라요' and '모르겠어요' has same meaning, and '모르겠어요' is a euphemistic expression.