Wieso das denn(is this a correct sentence, and what does it mean?) <- yes, it means 'Why is that?'. I have no idea why we add 'denn' because 'Wieso das?' is perfectly fine as well. i think Wieso das denn is easier to say haha

''Und Sie leben hier zu drite(How to translate this?) <- 'Und Sie leben hier zu dritt' 'And you live here with three people' 

Das ist geil!(is this mean ''this is cool''?) <- yes. (or, in a sexual context, 'satisfactory'. i dont know the movie haha)

Whar's the difference between ''Anschnallzeichen'' and ''Anschnallen''? <- were they in a plane? Anschnallzeichen is the light in the plane which turns on and off depending on whether you need to have your seatbelt fastened or not. 'sich anschallen' is the verb described above

'Hey, kannst du mir das mal zeign'''(Is this mean ''hey can you show me how to do that''?) <- yep. zeigen is written German

Ich vermiss ihr/ihm auch''(Should it be ''ihn'' or ''ihm'', and what's the difference?) <- Ich vermisse ihn auch. That is the only correct way because 'vermissen' comes with the Akkusativ case.

Was ist denn(it  literally means ''What is it'', but can it also translated as ''what is reason''?) <- Was ist denn is a rather aggressive question meaning 'whats wrong?'. If somebody keeps on staring at you, youd probably ask that person 'was ist denn?' 

what does  ''Das fass ich nicht'' mean? <- in written German, it's Das fasse ich nicht. literally, it means 'I cannot realise it' -> It cannot believe it, unbelievable

Duschlampe(I know Schlampe but Duschlampe? light bulb? or showers) <- hahaha, they say 'Du Schlampe' meaning 'you ....'

Du mich auch(??) <- very difficult to explain. there are two main ways of using it. first, if somebody insults you by telling you to do something 'f you!', you can respond that to shoot back. or if somebody wants you to do something and you think its ridiculous or outrageous, you can use it. 'Go to the supermarket and buy milk!' 'du mich auch'

Dunkeln (It means ''dark'' but, is it adjective or noun?)

Was it "Im Dunkeln"? 

Then it means "in the dark", and "Dunkeln" would be a noun.

Sentences:

  ''Was ist der Unterschied?''  (Is this sentence correct?) <- yes, it means 'What's the difference?'

''Ich schaff das nicht'' (When ''schaff'' is not in a sentence, can it mean ''can do''?) <- in written language, it's Ich schaffe das nicht which means I cannot do it.

 What's the difference between ''ihn'' and ''ihm''(They both mean ''him'') <- Different cases. ihm is Dativ, ihn is Akkusativ. I dont know in how far you've studied these.

''Das ist lieb gemeint'' (What does this mean? Thank you for your kindness? but ''lieb'' means ''love''...) <- Liebe means love, lieb is 'kind'. So it means like 'kind intention'. A 'but' sentence usually follows haha