Teaching must and must not modal verbs of obligation
It's useful to teach modal verbs of obligation comparing must and have to. But personally I'd teach them one at a time, I prefer to drill something thoroughly (using games for primary school kids, or teens, to make it interesting) and avoid confusion for life! Then do a lesson comparing them. If you teach too much new grammar in one go, students get confused and don't remember any of it!
To make this modal relevant to your primary school pupils or your teens, how about asking students to write a list of things they consider to be non-negotiable imperatives. Don't interfere with the content. Students may need some examples to focus, but it should be their list, not yours. These could be:
- Four essentials for a good party. The music must be good. My friends must be there. Students discuss their top four in pairs. Then vote as a class on the short-list and come up with the four top essentials, in order of importance. Use positive obligation for this.
- Students imagine they have two kids of their own. Vote on the top six cast-iron rules for these kids at home. If some children say they will never have kids, no probs, they can come up with a list of rules for how children in general. Use negative obligation for this. My children must not smoke in the house.
- The one absolutely non-negotiable obligation of a friend. To be my true friend, you must never lie to me. The class votes the top three after discussion in pairs.
I've already said this but be sure not to interfere with the content. It's their list, not yours. If it's their list and their priorities, they will be more interested in it than a list of obligations that resemble parental nagging. Having students come up with their own topics, whether at primary school or as teenagers is a good way to keep your lessons relevant to your students. Let me know how it goes in the comments below, and feel free to ask me any questions!
See my speaking fluency role-plays and skits for teens, coming soon, suitable for A2-B1 CERFA and High Novice-Low Intermediate ACTFL levels. There's a skit dedicated to modals of obligation. This is a free sample for the first and second conditionals.