Modal auxiliaries are auxiliary verbs that lend different shades of meaning to the main verb to which they are attached. Modals help to express the mood or attitude of the speaker, and in subtle but powerful ways they convey ideas about possibility, probability, necessity, obligation, advisability, and permission.
The single-word modal auxiliaries are:
| Can | May | Must | Shall | Will |
| Could | Might | Should | Would |
Some phrasal modal auxiliaries are:
| Be able to | Be supposed to | Have to | Ought to | Have better |
| Be going to | Be to | Have got to | Used to |
Single-word modal auxiliaries are always followed by the simple (or “stem”) form of the verb.
Here are some examples of how single-word modal auxiliaries influence meaning:
| Statement | Meaning | |
| I can go. | > | I am able to go. / I have permission to go. |
| I could go. | > | If certain conditions were met, I would be able to go. |
| I may go. | > | It's not certain, but there's a chance that I'll go. / I have permission to go. |
| I might go. | > | It's not certain, but there's a chance that I'll go. |
| I must go. | > | I am obliged to go. |
| I shall go. | > | I am going (in the future). |
| I should go. | > | It is advisable/recommended/obligatory that I go. |
| I will go. | > | It's certain that I am going (in the future). / I am willing to go. |
| I would go. | > | My going is uncertain; it depends on certain conditions. |
The following chart demonstrates some uses for single-word modal auxiliaries. Notice that in some cases when the verb tense changes, the modal is replaced with another word or group of words. Empty spaces denote non-existent forms.
| Modal | Meaning | Present Form | Past Form | Future Form | Negative Form |
| Can | 1. Ability | He can play the piano. | He could play the piano. | He will be able to play the piano. | He cannot play the piano. |
| 2. Opportunity | I can go now that I've finished working. | I was able to go when I finished working. | I can go when I finish working later tonight. | I cannot go. | |
| 3. Permission | Their parents say they can have a party. | Their parents said they were allowed to have a party. | Their parents will surely say that they can have a party. | They cannot have a party. | |
| 4. Informal request (refers to near future) | - | - | Can I borrow your book? | Can't I borrow your book? | |
| 5. Possibility (usually a generalization) | It can be tiring to study and work at the same time. | - | - | It can't be tiring to sit around and watch TV all day! | |
|
Could
|
1. Past of can | - | Before I hurt my wrist, I could toss pizza dough. | - | Even before I hurt my wrist, I couldn't toss pizza dough. |
| 2. Conditional | - | I could have made an omelet if we had had the ingredients. | I could make an omelet if we have the ingredients. | I couldn't make an omelet even if we had the ingredients. | |
| 3. Suggestion | - | You could have taken a long weekend to visit your family. | You could take a long weekend to visit your family. | - | |
| 4. Polite request (refers to near future) | - | - | Could you phone me when you arrive? | Couldn't you phone me when you arrive? | |
| 5. Possibility / Impossibility | Ms. Stevens could be the electors' choice for mayor. | Ms. Stevens could have been the electors' choice for mayor. | Ms. Stevens could become mayor. | Ms. Stevens couldn't be the electors' choice for mayor. | |
| May | 1. Permission | You may submit your paper via email now. | You were allowed to submit your paper via email. | When you finish, you may submit your paper via email. | You may not submit yoru paper via email. |
| 2. Polite Request (refers to near future) | - | - | May I use your phone? | ||
| 3. Possibility | The bakery may be open now. | The bakery may have opened earlier today. | The bakery may open later today. | The bakery may not open. | |
| Might | 1. Request (refers to near future) | - | - | Might I use your phone? | - |
| 2. Conditional | If you give me a chance I might impress you! | If you had given me a chance, I might have impressed you! | If you will give me a chance, I might impress you! | Even if you give me a chance, I might not impress you. | |
| 3. Suggestion | - | You might have called to say you weren't coming. | You might call to say you aren't coming. | You might not want to call to say you aren't coming. | |
| 4. Possibility | My dog might be the one who made the mess. | My dog might have made the mess. | My dog might make a mess. | My dog might not want to call to say you aren't coming. | |
|
Must
(Must not) |
1. Certainty | She must be the best athlete in our school's history. | She must have been the best athlete in our school's history. | - | She must not be the best athlete in our school's history. |
| 2. Strong recommendation | To be healthy, you must eat well. | To have stayed healthy, you should have eaten better. | To be healthier, you should eat better. | To be healthy, you mustn't eat poorly. | |
| 3. Necessity | Visitors must sign in at reception. | In the past, visitors had to sign in at reception. | In the future, visitors must sign in at reception. | Visitors don't have to sign in at reception. | |
| 4. Prohibition | - | - | - | You must not disrupt his concentration. | |
| Shall | 1. Inevitability | - | - | On day, we shall overcome our dependence on oil. | Humanity shall never abandon the quest for immortality. |
| 2. Promising (British usage) | - | - | I shall ensure that everything goes according to plan. | I shall not let you down. | |
| 3. Future action | - | - | Our train shall arrive ahead of schedule. | Our train shall not arrive ahead of schedule after all. | |
| Should | 1. Recommendation | For strong bones, you should drink milk. | For stronger bones, you should have drunk milk. | For strong bones, you should start drinking milk. | You shouldn't drink too much milk. |
| 2. Obligation | - | - | I should return this library book. | - | |
| 3. Expectation | She should be at the airport right now. | She should have arrived at the airport by now. | Sh should arrive at the airport before 9 o'clock tonight. | She shouldn't be at the airport yet. | |
| Will | 1. Future action | - | - | We will go to the movie tonight. | We will not go to the movie tonight. |
| 2. Promising | - | - | I will ensure that everything goes according to plan. | I will not disappoint you. | |
| Would | 1. Past of will | - | You said we would go to the movie. | - | You said we wouldn't go. |
| 2. Conditional | If I were in trouble, I would ask him for help. | If I had been in trouble, I would have asked him for help. | If I got in trouble, I would ask him for help. | Even if I got into trouble, I would not ask him for help. | |
| 3. Repetition in the past / habitual action or behaviour | - | We we were kids, we would always play street hockey in the alley. | - | When we were kids, we would not play street hockey in the alley. |
Like single-word modal auxiliaries, phrasal modals are always followed by the simple (or “stem”) form of the verb.
Unlike single-word modals, however, most phrasal modals contain the verbs be or have, which must be inflected to identify tense and number (I am, you are, he/she/it is, I was, you were, he/she/it was, etc). Exceptions are used to and ought to, which require no change in verb form.
Had better does not require inflection, and it does not include the word to as the other phrasal modals do. Like single-word modals, it is followed immediately by the simple form of the verb.
Here are some examples of how phrasal modal auxiliaries influence meaning:
| Statement | > | Meaning |
| I am able to go. | > | I can go. / I have permission to go. |
| I am going to go. | > | I will be going (in the future). |
| I am supposed to go. | > | It is understood/expected/agreed-upon that I will go. |
| I am to go. | > | I am expected/obliged to go. / I have been ordered to go. |
| I have to go. | > | I am obliged to go. |
| I have got to go. | > | I am obliged to go (extra emphasis on the obligation). |
| I ought to go. | > | It is recommended/advisable that I go. |
| I used to go. | > | I went on a regular basis (habitual action in the past). |
| I had better go. | > | It is advisable that I go. |
The following chart demonstrates some uses for phrasal modal auxiliaries. Notice that in some cases when
the verb tense changes, part of the modal is replaced with another word or group of words. Empty spaces
denote non-existent forms.
| Modal | Meaning | Present form | Past Form | Future Form | Negative Form |
| Be able to | Ability | She is able to join us. | She was able to join us. | She will be able to join us. | She is not able to join us. |
| Be going to | Plan (for the future) | - | I was going to start university next fall. | I am going to start university next fall. | I am not going to start university next fall. |
| Be supposed to | Expectation | - | He was supposed to bring dessert, but I guess he forgot. | He is supposed to bring dessert tonight. | He is not supposed to bring anything tonight! |
| Be to | Expectation / obligation | - | She was to see the project through to its end, but she left before its completion. | She is to see the project through to its end, no matter how long it takes. | She is not to see the project through to its end after all. |
| Have to | Obligation (=must) | I have to leave now. | I had to leave then | I have to leave in approximately 10 minutes. | I do not have to leave. |
| Have got to | Strong obligation | I have got to prepare my presentation now. | I needed to prepare my presentation at that time. | I have got to prepare my presentation as soon as I finish dinner. | I do not need to prepare my presentation anymore. I'm ready! |
| Ought to | 1. Recommendation | One ought to express one's gratitude. | We should have sent a note to our hostess. | We ought to send a thank you note to our hostess. |
We ought not forget. *to is dropped in the negative |
| 2. Certainty | When one studies hard, one ought to pass. | With all the studying she did, she ought to have passed her exam. | With all the studying she's done, she ought to pass her exam. | - | |
| Used to | Habitual past action / behavior | - | My grandfather used to walk 3 miles to and from school every day! | - | - |
| Had better | Recommendation | - | - | You had better go. It's getting late! | You had better not go. The weather looks pretty nasty! |
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For more information on modals and other auxiliary verbs, please refer to the following websites:
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