 Another point that students find difficult to grasp is that modal auxiliary verbs have no tense. To express tense or time, bearing in mind that they are not the same thing, you have to use or adapt the main verb. Some modals can't be used to talk about the past or the future. Let's have a look at this sentence. You must be in class by 9.30 every day. That's an obligation, an obligation for something that you need to do regularly. If you wanted to talk about the past, students might initially feel that you can write a sentence like this. You must were in class by 9.30 every day to express an obligation in the past. That is not the case. You might think that one way of approaching that could be. You must have been in class by 9.30 every day. Unfortunately, this is now not an obligation, but a deduction. So the modal auxiliary verb must, in this case, can't be used to talk about an obligation in the past. In this case, you would use a semi-modal. You had to be in class by 9.30 every day, with had to, used to express obligation in the past. Let's have a look at the difficulties with semi-modals using had to as another example. You must be in class. Change the you to he and the must stays the same. You have to be in class. Change to he and have to changes to has to. In this session, we've covered the essential points required for you to teach modal auxiliary verbs, but always consult any materials that are appropriate to your specific teaching point, because as you may well have picked up, what initially appears to be quite a simple structure, using the nine true modal auxiliary verbs, can create difficulties for a lot of students when they first start to learn these. Then once you start to introduce the semi-modal auxiliary verbs, the difficulties can only increase.