"We need to close the door."

Translation: Vi behöver stänga dörren.

[legresam]

Is there a way to know when we use the "att" before infinitive verbs?

[Lundgren8]

It depends on the verb, I think we’re planning on writing an explanatory text about this at some point.

[legresam]

Good idea, it would help a lot

[SorceressQuistis]

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.

[Arnauti]

https://www.duolingo.com/comment/5892805

[SorceressQuistis]

Okay, I really should have found that before. Sorry. Thanks!

[Arnauti]

Trying to spread the links everywhere as much as possible! Btw I just realized there's a link in there about this subject (infinitive with or without att now), so I'll add that one too: https://www.duolingo.com/comment/7075383

[gramphos]

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.

[Arnauti]

There's a topic about this now here: https://www.duolingo.com/comment/7075383

[MarisBoyd]

Is it actually incorrect to say vi behöver att stänga dörren?

[Arnauti]

Yes, it's incorrect in this case – never use att after modal verbs, behöver is one of them. There are many other verbs where both ways work though.

[MattTheBandGuy]

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.