5.4.1 Modal verbs are auxiliary verbs, which are a kind of ‘helping’ verb – they help the main verb to make the sentence. (See Lesson 4.6 Auxiliary Verbs.) While the three primary auxiliary verbs (be, do, and have) help the main verb to make the tense, modal auxiliary verbs help the main verb to express a group of meanings (or ‘modes’), including permission, possibility, necessity, and more.
5.4.2 There are nine modal auxiliary verbs in English:
5.4.3 A modal verb is always followed by the bare infinitive form of the verb (without to). For example:
5.4.4 Like other auxiliary verbs, a modal verb cannot be conjugated: it always keeps the same form and does not have an infinitive, s form, past tense, past participle, or ing form. To make the past and future forms of some modal verbs we can use other constructions, which are like modal verbs:
have to (which conjugates ‘must’)
ought to (which conjugates ‘should’)
be able to (which conjugates ‘can’ and ‘could’)
be allowed to (which conjugates ‘must not’ and ‘may’ and ‘might’)
‘must’ has no past or future forms, so we conjugate ‘have to’ to make ‘had to’ (past) and ‘will have to’ (future):
We cannot say: ‘I musted go to the bank yesterday.’ (past) or ‘I will must go to the bank tomorrow.’
‘can’ has no past or future forms, so we conjugate ‘be able to’ to make ‘was/were able to’ (past) and ‘will be able to’ (future):
We cannot say: ‘I canned buy a new dress yesterday.’ (past) or ‘I will can buy a new dress tomorrow.’
In fact, we never put two modal verbs side by side in a sentence:
Not possible: Possible:
She should must finish her work. She should have to finish her work.
I might can lend you my bike. I might be able to lend you my bike.
There are other options for the past and future forms of modal verbs, including ‘must’ and ‘can’. For example, the past of ‘can’ is often ‘could’ (see below). See also the free worksheet Modal Verbs – Revision Page for more information:modal-verbs-revision-page
5.4.5 Another construction which is connected with modal verbs is ‘used to’. We can use it instead of ‘could’ with the use of past ability, for example:
When I was a child I could play the guitar really well.
When I was a child I used to be able to play the guitar really well.
‘Used to’ uses the auxiliary verb ‘did’ to make negative and question forms, and follows this pattern:
5.4.6 We often use a question tag at the end of a sentence to try to get a quick confirmation from another person or keep the conversation moving:
You sent that email, didn’t you?
If the preceding sentence is positive, the question tag will be negative, while if the sentence is negative, the question tag will be positive. For example:
They didn’t want any pudding, did they?
They wanted pudding, didn’t they?
The question tag always matches the tense in the preceding sentence, so we can’t say, for example:
Your sister likes pizza, didn’t she?
(present simple) (past simple)
We use modal auxiliary verbs in question tags in the same way as other auxiliary verbs. For example:
We’ll find out the results on Tuesday, won’t we?
We can’t stay for long, can we?