Many students often get confused between the modals of obligation (should, must and have to) and it can often be a little tricky to teach.
Now, I am not saying this will suddenly make it as easy as ABC, but I hope it helps.
The PowerPoint gives illustrated examples of the three modals of obligation (both the positive and negative forms) for you to use to teach the class. There is then a worksheet, simply asking the students to match the modal with the picture.
Going back to the PPT there is a class activity. Split them into groups and give them each an instruction manual to write. They should tell people what to do the situations, using the modals of obligation. The instruct manual titles include "Driver". "Money", "University", "Building" and "Baby". This might be too advanced if you are teaching a lower level class. But will work well for a higher level class, of teenagers for example.