How to use modals of obligations

Welcome to today’s Lunchtime Lessons post. This week we are looking at modal verbs of obligation. Our Lunchtime Lessons are free Online English classes where we look at areas of English which are often difficult for students.​

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Modal verbs
Welcome to this week’s Lunchtime lesson.  Today, we’re going to talk about modal verbs of obligation. So, first of all, what is a modal verb? A modal verb is a special type of verb that changes or affects the other verbs in the sentence. In other words, it gives some extra meaning. They’re used to show the level of possibility or to indicate ability or to show obligation or give permission. As you can see, modal verbs have lots of different functions. On top of that they also behave differently to what we would call an ordinary verb. So let’s take a closer look at them.
If you’ve studied modal verbs before, it was probably presented to you as grammar. Today, we are going to approach them differently. We’re going to think about them as vocabulary. I think this is a good way to learn a lot of English topics that are usually taught as grammar. If we think about it as vocabulary, we can think more about the function of the structure and what it’s allowing you to express, rather than thinking about how you should be constructing it. So, I want you to think that a modal verb is a bit like a paintbrush. You’re going to dip it in different colours and you’re going to use it to paint your sentence with a different meaning.
Let’s take an example sentence: I call my mother. By adding a modal verb, I can express a different meaning. I can express that there is an obligation to call her 


  • I must/have to/need to call my mother. (obligation)

or that it would be a good idea to call my mother, but if I don’t do it’s OK too.  

  • I should/ought to call my mother. (good idea)

Do you realize what those modal verbs have done to our sentence? The modal verb has painted our sentence with a new meaning of obligation.

The most important thing, like always when learning language, is context. Your context is key. Modal verbs will change their meaning depending on the context. Lets look at an example:

  • I must call my mother. Must is expressing an obligation.
  • My mother must be busy, she isn’t answering the phone. Must is expressing a deduction or speculation.

Today, we are going to focus on modals for obligation. There will be another session about modals for deduction and speculation in a few weeks.
Modal and semi-modal verbs
When talking about modal verbs, you’ll also hear something called a semi-modal. It’s not what we would call a proper modal. Let me show you the differences.

Modal verbs:

  • are always followed by a bare infinitive. A bare infinitive is without ‘to’. I must go. I never say I must to go. Remember this, as it a really common error.
  • can’t be changed into a past tense. I can’t say I musted go. There is one exception: should.

Semi-modal verbs have the same function as a proper modal verbs, but:

  • can be followed by ‘to’ – I have to go
  • can have a past tense – I had to go

Modals of obligation
Here is a list of modals and semi-modals of obligation. I’m sure you’ve seen most of them before. An important side note here, might or may are NOT modals of obligation. We’ll be talking about those in another session.

Considering our modals of obligation, it is important to know what they express. Can you sort them correctly? Which one would go to the top where there’s a 100% obligation and a consequence? Which one would go in the middle and are advisable? And which are on the bottom as modals of obligation where there is no consequence? To see the solution press on square 2.

Common mistakes
Let’s look at some areas related to modals of obligations where learners often make mistakes.
We already said, there is no past tense of must when we’re talking about obligations. If you want to speak about the past you need to use a semi-modal like had to or needed to. When you check our scale from before you notice they express the same grade of obligation as must.
Be careful with negatives. What I mean by that, the opposite of must in terms of obligations is not mustn’t. Must and mustn’t are equal in terms of obligations. As an example:


  • You must pay your taxes. If you don’t you’ll go to prison.
  • You mustn’t kill someone. If you do, you’ll go to prison.

In both cases there are consequences. So, making a modal verb negative doesn’t make it an opposite in terms of intensity.
Need to is very often used instead of must or have to. I recommend trying to bring that into your vocabulary. Native and proficient speakers use the term need to all the time whereas students hardly ever use it. The same with can’t, be allowed to and mustn’t. Students often stick to mustn’t whereas native speakers use can’t and be allowed to more often.
As mentioned before, should can be used with a past aspect. For example: I should have called my mother (should + have + participle) means I didn’t call her but it would have been advisable if I did. The important thing about using should in this structure is the pronunciation. We’re going to contract the verb have because it’s an auxiliary verb here and it’s not important for the meaning of what we want to communicate. And because it’s not an important word, we’re going to change the vowel sound from /æ/ to a /ə/. So  should have called becomes one block of sound: /ʃʊdəvˈkɔ:ld/. When we talk about our modal verbs of deduction, we’re going to do a lot more work with this pronunciation. As I’ve told you many times before, people will understand you if you don’t use the contractions. It is more for your own understanding. Remember: If you can produce a sound you can hear a sound.
Another thing that I’ve noticed with should is that a lot of students are using it incorrectly. Should expresses something that is advisable. It is not a strong obligation with a consequence like must. Remember, if there is a consequence, you need to use a model or a semi model, which is strong in terms of obligation.
Another important thing when using modals is the register. A grammar book will tell you, that must is used for internal obligations (I must call my mother – something that I need to do) and have to for external obligations (I have to go to work – someone else is making me do that). But I really disagree with that. I think must is more formal and when you’re using it in your speech, it sounds much more serious. I recommend using have to instead of must when you’re speaking. The same applies to should and ought to. Ought to is a more formal way to express something that is advisable than should. And a third example is have got to and have to. Both mean the same but have got to is more informal and more commonly used in spoken English.
And now it’s time to practise those modals of obligations. If you like you can leave us a comment and share some advice or tell us about an interesting obligation.

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