"We need to close the door."
Translation:Vi behöver stänga dörren.
It depends on the verb, I think we’re planning on writing an explanatory text about this at some point.
Have you considered doing a section for the most frequently asked grammar questions? Everyone is asking similar questions and you and the others are replying individually most of the time. It would save your own time greatly to have a standard grammar section to refer us to for the most common queries.
Here "behöva" is used as an auxiliary verb, "hjälpverb" (helping verb). The main verb that follows an auxiliary verb will always be infinitive without att.
It makes perfect sense! In English, we would not say "We must to close the door." We would say "We need to close the door." I can see how some verbs in Swedish work the same way. As learners of Swedish, we just need to learn those verbs and user them regularly in practice.