Modals Mustn’t, Don’t Have To, Must, and Have To: The #1 Reason Why Students Struggle

When I first started teaching English as a language, I came across an intermediate lesson that left me a bit dumbfounded. I was walking in to teach what we often group together as ‘modals of obligation’ for the first time, and it seemed pretty straight forward. I knew what must, mustn’t, have to, and don’t have to meant, and the book had some cute activities. What could go wrong?

Well, the book was teaching mustn’t with don’t have to, and must with have to. The negative form of the modals were together, and the positive forms were together. And while that set up makes sense in some ways, it was really confusing to students. This presentation assumes that positive forms and negative forms naturally contrast with each other, which is not how meaning works with these modal verbs.

“Teacher, if mustn’t is negative, then the opposite is must, right?

Grammatically, yes. Contextually? No. Mustn’t is not the opposite of must. These modals express two different ideas, and I needed to take a minute and figure out a better way to explain that.

Since it was a class of adults, we took a tea break, and I sat down for 15 minutes and wrote out how I used these modals. Then, I saw the underlying issue and what was causing my students’ confusion. This moment eventually became the foundation for how I now teach modals of obligation.

The problem is this: the modals must, musn’t, have to, and don’t have to are often taught in a way that completely disregards the contexts in which native English speakers use these modals. These boxed curricula and textbooks sometimes have it wrong.

So let me show you how I have come to teach these modals, eliminating a whole lot of confusion and allowing for students to increase their fluency by using these modals correctly, in context.

1. Start with Mustn’t

The problem is that mustn’t is often paired with don’t have to, which means not necessary. For this reason, students often make the mistake of thinking that musn’t means the opposite: necessary. This is not true.

Mustn’t means something is forbidden or prohibited, and it almost always comes from the outside. These are rules, laws, or strong cultural norms that tell us “do not do!” These are also easily understood across most cultures when you introduce signs that indicate forbidden or prohibited actions, and this is why my modal unit begins with mustn’t.

Modals Mustn't, Don't Have To, Must, and Have To: The #1 Reason Why Students Struggle — *This is an example from my uni*t *when I introduce mustn’t*.

Examples should look like 👀

Students mustn’t use their phones during a test.
You mustn’t drive above the speed limit.
You mustn’t swim at this beach.

2. Follow up with Have to

Once the students understand that mustn’t is something that is prohibited from an outside source, it only makes sense to introduce students to have to. While mustn’t tells us what is not allowed, have to tells us what is required.

Have to means something is an obligation or requirement. It also comes from the outside which means it is used for rules, laws, or strong cultural norms as well. However, have to communicates, “do this!”

Examples should look like 👀

Students have to wear school uniforms.
You have to drive with a seat belt on.
You have to wear a swimsuit at this beach.

3. Finish with Must & Don’t Have to

Now that students know the modals associated with outside rules and laws, it is an easier shift to look at the modals must and don’t have to.

Must means something is necessary either from an outside or inside source. If it is coming from the outside, it is usually a requirement from someone who is in a position of power over you: think your boss, a teacher, even a doctor or dentist.

If it is coming from the inside, must simply means that it is necessary or important to the speaker.

Examples should look like 👀

You must finish this report before the end of the day.
I must do my laundry if I want to wear my favorite clothes.

Don’t have to means something isn’t necessary for the speaker. It simply means there is no requirement to get it done.

Examples should look like 👀

I don’t have to finish this report today. It isn’t due until next week.
I don’t have to do laundry because I have a few clean clothes in my closet.

💡Teaching Tip for ESL Classrooms

Use well-known laws, school rules, home tasks or chores.
Have students sort or identify sentences as forbidden (mustn’t), obligation or requirement (have to / must), necessary for self (must), or not necessary (don’t have to).

Understanding modals of obligation helps students navigate rules, obligations, and social norms in English. Once learners see the clear difference between obligation, prohibition, and necessity, their accuracy improves dramatically. No more confusion, no more, “I don’t have to smoke here.”

That’s why I teach these modals in this order and why this framework is built directly into my modal unit.

If you’re looking for a ready-to-use way to teach must, mustn’t, have to, and don’t have to without student confusion, this approach is designed to support both you and your learners.

If you’re also looking for a way that explains these modals, perhaps you want clarity before you teach them or your students need another explanation, then please check out my YouTube videos as well.