"He was forced to wait."
Translation:Han blev tvungen att vänta.
Hard to say for sure. Duo's had a bug previously where it forgot to teach you a word or two during an initial lesson, only to carry on as usual afterwards. Maybe that's what happened? There's also the problem that if you have two synonymous words and you've learned one, Duo occasionally just assumes you know the other one as well. Some languages have a fair number of terribly annoying multiple-choice questions because of this.
It seems that there are sometimes two versions of the same lesson, one the normal lesson and one that looks like a review. Sometimes the review lesson pops up before the normal lesson, which means you are asked multiple-choice questions that you don't yet know how to answer. If that happens, exit the lesson and restart. Usually (90%+ of the time) this will put you back in the right place. I've also had to deal with this in the Russian course.
Did you read the Tips & Notes for this section? https://www.duolingo.com/skill/sv/Verbs%3A-Modal (may not be available if you're on mobile)
Basically the mode in modal is like English 'mode' meaning 'way'. Modal verbs are verbs like vill 'want to', måste 'must' and so on. They're usually combined with another verb, like here in the infinitive (att vänta). They add a certain kind of 'how' to other verbs. Like, eat is neutral, but want to eat is a special modality.
That's pretty much all there is to it, but they tend to look different than normal verbs – normal verbs would end in -ar or -er in the present tense, but verbs like vill and måste of course look very different.