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Modals of Obligation

Obligation modals, while simple to use, can be tricky for A2 level students. For example, have to and don’t have to while on the surface seem like opposites, they mean entirely different things, unlike must and mustn’t. There is also the concept of internal vs external obligation, yet must and have to frequently overlap here, particularly when the these modals are used in writing, in the negative form, or in the past tenses. Indeed, there are several pitfalls students need to avoid when using these modals, particularly have to and don’t have to.

Visual Aides

In a traditional classroom setting, I use the whiteboard to introduce these modals. Students at the A2 level are usually already familiar with them, so at this point I can use elicitation to get the words on the board. I do this by drawing a 2×2 grid, labeling the Y-axis either positive or negative, and the X-axis internal and external. The final result of our collaboration usually looks something like this:

2×2 Grid layout of obligation modals ©2020 Robert Spoden

While this layout looks confusing at first, it usually makes sense to students while it is being constructed. First, I explain the difference between internal and external – I attempt to elicit the difference between I must lose weight and I have to lose weight. Usually someone will figure out that the first statement is that I want to go on a diet to feel better, and the second is that a doctor told me to do it for health reasons. I go over a few other examples in this fashion to instill the difference between internal and external obligation. But then, it gets confusing because while the positive usage of these modals is straightforward, the negative is not – for example what if I am forbidden to smoke?

Negative obligation

That’s where the diagonal arrow in the middle comes in, as well as the red box around don’t have to. Negative modals are confusing for students because the line between internal and external obligation becomes fuzzy. Here I emphasize that don’t have to is completely different, in that it means that something is not necessary, that doing the action attached to don’t have to is optional. Don’t have to, while on the surface is the negative of have to, actually stands on it’s own and has almost nothing to do with obligation, while have to has everything to do with it. So how do students produce negative external obligation? Here, I elicit for mustn’t. Sometimes, students will come up with can’t, and while this technically works mustn’t is the proper answer. The diagonal arrow between have to and mustn’t serves as a visual aide to this end.

written external obligation

The blue box around must and mustn’t serves to help students understand the special case usage of internal obligation modals when they are written in a set of rules or laws. The example I like to give is the about the seatbelt signs commonly found in the US and other places.

Seat Belts Must Be Worn Label K2503 - by

Here, I usually do a short segue into the passive voice, because usually when rules are in written form using must, the passive voice is used. Students at this point in the journey have likely been introduced to the passive, and if some are still struggling with this concept I make available some remedial worksheets about this topic on an asynchronous platform such as Edmodo or Google Classroom, and invite students to email me questions if they have any.

The main point I try to drive home is that must and mustn’t are used in writing when communicating external obligation, and have to is usually reserved for spoken external obligation.

past tenses

The other purpose of the blue box around must and mustn’t is to emphasize that these modals cannot be used in the past. This again blurs the line between internal and external, because in the past we use have to for both in the positive, and couldn’t in the negative.

Another caveat to draw attention to is subject-verb agreement on have to / don’t have to. Students will usually get this one wrong in their first attempts at production. It’s confusing because must doesn’t change but have to does, and they both take a bare infinitive verb. Students will often try to make that verb agree with the subject when they use have to, and here it is useful to try to break that habit.


The presentation of this grammar may be overwhelming to some students because there are various nuances and loopholes. It’s useful at this point to give students the opportunity to dive in and get their hands dirty. I have found that a couple activities here are useful. The first activity involves students being put into small groups and coming up with rules for the classroom. One of the students writes down the rules to aid in remembering them. When I monitor the groups, I don’t interfere or try to correct them at this stage for I want them to get comfortable with making attempts in spoken production of this grammar. After about 5 minutes or so, the groups all report back to the class their sets of rules, and I take the opportunity here to correct any errors, using the whiteboard to write out each rule as it’s said and eliciting from other students in the room where the possible mistake is and why, or if what was spoken and written is fine. I add a few rules of my own, some sprinkled with common errors and see if the students can catch and correct them.

In the second exercise, I again break out into small groups, but this time I have them come up with rules set in the past when they were children. I make sure to encourage students to use don’t have to as well so as to give them sufficient practice with conveying lack of necessity.


There are a couple of tools I like to use to reinforce this grammar. One of them is Kahoot, and the other is a short video I found on YouTube which mostly explains what was presented. There are several decent Kahoots about this topic with varying degrees of difficulty. Here, I usually have the students play a round, and if a significant number of students get the question wrong, we pause the game and I attempt to elicit an explanation for the right answer from the class. Doing a Kahoot is quite useful, particularly on the next class day as a warmer so as to refresh students’ knowledge of this grammar.

The second tool is a video called Grammar Game Show which was produced by BBC. Here’s a link to it:

Grammar Game Show ©BBC Learning English

The Grammar Game Show series of videos covers a wide variety of topics and is quite a useful resource. I like this format because I can pause the video after Will, the host, asks the contestants the question to see if students can come up with the answer. The explanations offered in this video are also a great source of review of what was presented in the classroom, and the video itself can be posted on an online learning platform for students to review at their leisure.

further considerations

Must and can’t are also used to talk about certainty, for example: “Carol must get very bored in her job. She does the same thing everyday,” or “You’ve just had lunch. You can’t be hungry already. This usage might confuse the students, but if I am dealing with a stronger class, I will introduce these concepts in order to further expand their knowledge of these modals. There are many resources online for this grammar. One of my particular go-to resources is a book called English Grammar in Use by Raymond Murphy. Here is excerpt of a page from this book that describes this usage.

English Grammar in Use. Raymond Murphy. ©2004 Cambridge University Press

This book is widely available in Turkey and I encourage my students to buy it so that they may do additional exercises on the grammar we cover in class.

Well, this about wraps things up. Obligation modals, while fairly straight-forward on the surface have many nuances and present a lot of potential pitfalls for English learners. It is important to carefully guide students through this maze of convention, as the rules get quite blurry in some places. As always, practice makes perfect, and making use of an online platform such as Edmodo or Google Classroom is quite helpful in this area so that students will have lots of opportunity to get comfortable with this grammar.