The truth about how “must” and “have to” are actually used

Modal verbs can be a difficult area for students. They are used for ability, advice, permission, probability, obligation, and other things like speculating about the past. We do these things all the time so modals are very common in English. They are also often tested in exams, e.g. when describing a picture, or giving recommending something to the examiner. In this article, we are going to focus on two that are used for obligation: the pure modal “must” and the semi-modal “have to”. For students, learning the difference between them is not easy and you might not feel confident about using them in a natural way - some students even try to avoid them completely! Unfortunately, English coursebooks and grammar websites have not always helped, sometimes giving incorrect information or advice that just isn’t true or helpful. Luckily, our experts at ABEL consultants are here to help you advance your English with some top tips on the truth about these modals.

Is it important that “have to” is a semi-modal?

Well, we think it is! Some online guides say that “have” is a main verb in this context, which is really upsetting to the grammar geeks at ABEL - it’s not a main verb. In the sentence,

“You have to share.”

“share” is of course the main verb, because it carries the meaning. We can take away “have to” and the sentence still has meaning, we just sound like a neanderthal at a dinner party “YOU SHARE!”. This is because “have to” is just an auxiliary (helping) verb. In fact, “must” is also an auxiliary. The fact “have to” is a semi-modal is important because it tell us we can do things to it: we can put it in different tenses (had to), third person “has to”, and to make the negative or question form, we need “do” (do I have to share?). Notice, it’s not “haven’t to”, a typical mistake students make. You should treat “have to” like any other verb you are conjugating first in a sentence. Easy, right?

We don’t say “musted “musts” or “do you must?”, because it’s a pure modal. For examples of other modals, have a look at the British Council’s guide. (In this article, we will continue to refer to both “must” and “have to” as modal verbs, but remember, you know the truth!)

What’s the difference in meaning?

The good news is that you can use the positive forms of these modal verbs interchangeably without any big misunderstandings. If we substitute “must” into the example above, the meaning is still more or less the same.

“You must share.”

Both sentences are using “must/have to” to indicate that there is a strong obligation to share, or that it is necessary. (When the obligation is weaker, perhaps just a recommendation, you can use “should”). Some websites and course books will give additional rules about the differences between the modals’ positive forms “must” and “have to”, but we will discuss below why these aren’t really so helpful and why you might want to avoid them. 

Typical places to see “must/have to” are on lists of rules, signs and notices, e.g. “Eye Protection Must Be Worn At All Times”, or even to convince people to read an article you’ve written…

What students find more difficult is the negative forms of these modal verbs. Because if the positive forms have similar meanings, then the negative forms “mustn’t” and “don’t have to” will have similar meanings too, right? Wrong! 

“Mustn’t” means something is prohibited, or not allowed. Whereas, “don’t have to” means it’s not necessary, there is to obligation to do it. Here’s an easy example to remember this: if you learn English in a language school, or online, there is a good chance you don’t have to wear a tie. At ABEL consultants, for example, we care more about what our students need and what our consultants know, than whether anyone is wearing a tie. But it’s not prohibited. If one of our consultants wants to wear a tie, we aren’t going to tell them to remove it (unless maybe it’s an absolute fashion disaster...). 

However, if we were working in a factory, with dangerous machinery that your tie could get caught in, then we need to change our rule to,

“You mustn’t wear a tie.” 

(Because it’s dangerous, you might be pulled into the dangerous machinery, and it's not the main reason, but we’re sorry to say, but it doesn’t go well with that jacket...)

So what’s the problem with some of these coursebooks and other grammar websites?

The problem is that they often give rules to clarify the difference between “must” and “have to” which aren’t actually very useful. 

Here are some examples that we found online:

For general obligations, we use “have to” for general obligations and for specific obligations we use the modal verb “must”.

“I have to walk the dog every day after school.”

“I must tell you about what happened at the weekend.”

Confused? We are too. We agree that “I must walk the dog every day after school” sounds a bit unnatural, but “I have to tell you about what happened at the weekend” sounds perfectly fine, maybe to some proficient speakers it even sounds more natural. 

What about this one, perhaps even more confusing:

For internal obligations, we use the modal “must” and for external obligations, we use “have to”. 

“I must stop smoking, because it’s costing me so much money.”

“I have to stop smoking, because my doctor said it was affecting my health.”

Just like before, substituting “have to” in the first sentence sounds perfectly natural.

This isn’t an ideal situation. Forget that it’s probably very unlikely that you're going to think about “internal” and “external” obligation as an easy way to remember which words to use. But, in practice, the rules aren’t even true!

Is there an easier solution?

Yes! Absolutely. We’ve already said that the meanings of the modals’ positive forms are very similar and that the meaning is clear whichever one you use. But if you want to go deeper, and advance your English by being as natural as possible, then you need to:

• Forget these unhelpful rules

• Focus on frequency

By frequency, we mean: how often are each of them used? We don’t care about the past. We don’t care which one Charles Dickens preferred. We want to know, right now, how many people are using “must” and how many people are using “have to”? Luckily, we can find this out using something called corpora. These are collections of written and spoken language that I discussed on my blog about using research to improve teaching. Here is what the research tells us:

• If we battle “must” and “have to/has to” in various corpora (BNC/COPA/iWeb) we see that there isn’t a huge difference in how often they are used, roughly 50:50.

• But! And it’s a big but! According to the excellent research by Mike McCarthy and his team using the Cambridge International Corpus, when we use “must” we are only using it for obligation 5% of the time! Another 5% is just for collocations in expressions like “”I must say/admit”, and the other 90% is in its predictive or deductive sense, e.g. “You must be tired.”

Because “have to/has to” are only ever used for obligation, that means that (in speech) people are choosing it and avoiding “must” approximately 95% of the time! This research is also supported from my personal experience, particularly when training new language teachers: I often hear them say things like “I don’t really think I used ‘must’” or “we don’t really say must in my country, it sounds unnatural to me”.  

Coursebook after coursebook and website after website explain these modal verbs, attempting to make tiny distinctions about how we use them, but so many fail to mention or stress this extremely important fact that “must” is, or is becoming, unfashionable. 

So the easy and practical things to remember are:

• if you are trying to say something is prohibited, use “mustn’t”.

• If you want to say it’s not necessary, use “don’t have to”.

• And if you want to sound natural in English, use “have to”. Try to avoid using “must”, but if you must, don’t do it too much, or people might think you are unfashionable! Like this guy...

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