Grammar

So I think I just had my worst class last night. I never thought teaching grammar could be so difficult. Last night I was teaching the grammar point mustn't, don't have to, have to, should, shouldn't and must. All of my students were fine at first and they understood it, but when I did an exercise for them to practice, my whole class changed. I went over the answers and about half of them got one answer and I said another. I tried to explain why it wasn't the one answer but then they all started to argue and discuss in czech why. It was crazy and for some reason I confused them all. I tried to go through the grammar in the back of their book to help them,but they were taught differently. I gave them more practice, I hope that they will understand. I am not sure what i said that was so different or what confused them. I hope on Wednesday we can go through it again and clarify any confusion. GRAMMAR!!!!!!