If you say 'I don't need something' in Dutch that will be like 'Ik heb NIETS nodig', if you say 'I don't have milk' that will be 'Ik heb GEEN melk', and if you say 'I don't know' that will be 'Ik weet het 'NIET'. The negative words are in capitals, I hope this was a help, if you have more questions then don't hesitate to ask.
This sentence is possible, but in this sentence you put the stress on 'boeken'. In the sentence 'Jullie hebben geen boeken nodig' you put the stress on 'geen'. I would rather use 'Jullie hebben geen boeken nodig' because that sounds better, I hope I could help you with this explanation, if you have further questions, don't hestitate to ask me ;)
"nodig hebben" sounds a lot like "be(/have) neccessary for -jullie- to (have)"
you can't employ that for an answer though, right?
(edit: or it's also just because I mistakenly used singular on books) (which might be because "geen" stays the same for plurals, I'll get the hang of it :>)
what is the difference between you guys dont need books and you all dont need books... first i wrote you guys dont need books it said correct answer was you all dont need books then i tried you all dont need books it said correct answer was you guys dont need books... how is that even possible? a glitch in duolingo?