I understand the confusion. However, keep in mind that languages do not always use verb tenses and verb voices exactly the same. So there is hardly ever a one-to-one mapping of verb structures between two different languages. This is exactly the confusion here since the passive voice is employed a little differently in Dutch than in English (past tenses are another example). You would also use the passive a lot more in Dutch, where English would use some other structure to achieve the same meaning in a similar description. So sometimes one-to-one translation of the verb structure would not be right, while other times it may be correct. I know I'm not making it any easier, but keep this in mind when doing translation of tenses and sentence voices. You will develop a better feeling of what to employ in certain contexts as you progress. Time and practice are the keys here :)
This was a dirty trick, but essentially vanavond refers to tonight, rather than last night (gisteravond), but if you have been following the course I don't think you could have known that. In English, you can say tonight to mean something that happened already, but it is not so true in Dutch.
It might be best translated in the future, and all these comments are helpful, but it would be helpful if this grammar were clarified before a particular section. There are too many idiosyncratic constructions in this section without good explanations. I feel as though just a section for zijn in the passive and just a section with worden in the passive would have been more constructive and clearer.
Having been a contra dancer, and having gone to dance camps, this sentence makes perfect sense. "You'd better go take a nap, because there is a whole lot of dancing this evening.) We can use present tense for future actions in English, and often do. This sentence was my AHA moment.