3.4 The range of functions of modal auxiliaries
To summarise, the main modal auxiliaries are can, will, shall, might, must, dare, need, ought to, could, would, should, may, got to and have to. Many modal verbs can be used in two ways, one indicating how far the speaker thinks something is likely, the other indicating how far they think something is desirable or necessary.
The table below shows how you can use the same modal auxiliary to express different ideas. You don’t have to remember all of these; they are just here to show you some of the ways in which modal auxiliaries can be used to alter the meaning of a sentence or statement:
| Possibility/likelihood/certainty | Desirability/necessity/permission/obligation | ||
| logically necessary | He’s not answering – I guess he must be out. | compulsory | Construction site: Hard hats must be worn. |
| predictable | Who’s that at the door? Oh that will be Mariam. | determined | Whatever happens, I will wait for you. |
| probable | How long do the beans need? 20 minutes should be enough. | desirable | Some people think marijuana should be legalised. |
| possible | Careful – it could/might explode! | desirable | If you’re going out, you could/might get some milk. |
| possible | Tarmac can/may get very hot during summer. | permissible | You can/may take photos as long as you don’t use a flash. |
As is often the case with language, the difference between modals can be a ‘grey area’. There is no firm dividing line between what will happen, what may happen, and what might happen, nor between what you must, should or could do.