Third period, firts class with advanced I

Yesterday, I had my first class with the advanced English I. I got on time and I was ready to teach my class. It was almost four and just one student was in the classroom, and the teacher told me to start. I felt it strange because just one student was there. My original warm-up activity was the one called "broken phone" but how was I suppose to do something like that with one student. Fortunately, I had another activity, and it was about giving students pieces of paper with words, so they could form sentences with them. I gave that student the papers, so he could do what I asked him to do. He was about to start working on it when the other students got into the classroom. I asked them to make trios then and explained the activity again. I had to do the presentation of the topic by using a short reading. There students were to find out some sentences that were in passive voice which was the topic I had to teach. They did it by underlining the sentences. Then I continued with the grammar points. I had to explain on how to change an active voice sentence into passive one; it was a little bit hard because I had to include all the tense, so I was explaining each of them by using a chart I created in Audio Visual Aids. While explaining, I also involved students by giving more examples and have them change the sentences into passive voice. It was great because when I said the sentences almost everybody was giving me the answer. There some moments in which I asked them individually. Evething was going ok until a student asked a question regarding the future perfect, and it was about the position of the word "not" in the passive voice. I was sure of the answer I provided, but then I was analysing the sentence and now I know I made a mistake. I hope the teacher in charge explained it because I left after I finished my class. The feedback has not been given and I am sure the teacher will tell me about the mistake I made. In general I can say the class went well just the fact of the mistake I made.