Modal Verbs

I committed a mistake on the 10th question on the second quiz.

 The question goes like this, "The rock band _________play very well last year. Now they are much better. "

          I got confused between options 'couldn't' and 'can.' I was thinking, since there's the phrase 'much better,' they must have played good the previous year.

I also answered the third question of quiz three wrong. I didn't finish reading the whole sentence, and so I didn't know the significance of 'must.'

Modal verbs are quite confusing, but with practice, we'll be able to use them with ease. We use different kinds of modal verbs depending on the intensity of the situation or how we want to express ourselves. We use modal verbs to talk about our in/abilities, to tell if things are possible or not, to offer, to ask permissions and to make requests.