"You do not have to agree."
Translation:Du behöver inte hålla med.
You guys are arguing about the meaning of the negative (must not/need not) rather than the positive (must/need). With most English verbs X, the negation is equivalent to 'it is not the case that X'. For example 'You need not worry about it' = 'It is not the case that you need worry about it'. But English 'must' is unusual. 'I must' = 'I have to'; but 'I must not' does not equal 'I don't have to'. Rather' I must not' means 'I am not allowed to'. However, in Swedish, 'måste inte' does not mean 'I am not allowed to' but rather "I don't have to'. (Cf. similar German 'muss nicht'.) (Someone correct me if I'm wrong.)
Yes, I think so. Same for don't need to. It seems to me that English used to distinguish between need and have to much the same way we still distinguish between behöver and måste, but that the distinction has collapsed in English today, or at least in many versions of English.
Some verbs take att and some do not. I can't help with which - I get it wrong all the time - but the hints in the infinitive sections have good information. Getting it to stick is another thing entirely ...
Edit: Here's the link: https://www.duolingo.com/skill/sv/Verbs%3A-Infinitive