Advanced · Grammar Reference
Unit 4 — Modal Auxiliary Verbs
can · modals in the past · obligation/advice/permission/ability · softening
4.1 can
We use can to express what is generally and all-time true, and logically possible.
Cycling in town can be dangerous.Can cannot be used to predict future possibility. We must use will be possible or will be able to.
In years to come it'll be possible to have holidays on the moon. We'll be able to travel by space ship.4.2 Modal auxiliaries in the past
All the modal verbs given above are also used with have + past participle (the perfect infinitive) to express varying degrees of certainty about the past. Will have done is the most certain, and might/could have done is the least certain.
| Modal + perfect infinitive | Example |
|---|
| would have been | 'I met a girl at your party. Tall. Attractive.' 'That would have been Sonya.' |
| must have been | It must have been a good party. Everyone stayed till dawn. |
| can't have | The music can't have still been at dawn. Nobody danced. |
| should have been | Where's Henry? He should have been here ages ago! |
| may have got | He may have got lost. |
| might have decided | He might have decided not to come. |
| could have had | He could have had an accident. |
📌 Note
Can can only be used in questions or with
hardly, only, or
never:
Where can he have got to? They can only have known each other for a few weeks.4.3 Other uses of modal auxiliary verbs, present and past
Obligation
- Must/have to express strong obligation. The past is expressed by had to.
- Must can express an 'internal obligation', based on the speaker's opinion: I must get this jacket cleaned soon.
- Have to expresses an external obligation, based on rules and regulations or another person's authority.
- Mustn't expresses negative obligation. Don't have to expresses the absence of obligation.
- Absence of obligation can also be expressed with needn't.
- Needn't have (+ past participle) expresses an action that was completed but wasn't necessary: You needn't have bought any butter. We've got lots.
- Didn't need to (+ infinitive) expresses an action that was not necessary, but we do not know if it was in fact completed or not.
Advice
Should is used to express advice, or milder obligation. The past is expressed by should have (+ past participle).
You should rest. You should have taken it more seriously.Permission
- Can, could, and may are used to ask for permission. May sounds more formal than could. Might is extremely formal or tentative.
- The past of may and can is expressed by was allowed to. Could can only be used to report permission.
Ability
- Can is used to express general ability; the form in the past is could.
- To express a particular ability on one occasion in the past, could is not used. Instead, was able to or managed to is used.
Refusal and willingness
- Won't expresses a refusal. The past is wouldn't.
- We also use won't for inanimate things when we see their incapacity to function properly as a form of 'refusal': The car won't start.
Habit/Characteristic behaviour
- Will is used to express habitual or characteristic behaviour. The past is expressed by would.
- If will is stressed, it suggests criticism and irritation: David will leave his homework until the last minute. It's infuriating.
4.4 Softening the message
We can 'soften our message' and sound more polite and indirect by using certain constructions.
- A past tense: I wondered if you were free tonight? I thought we could go to the cinema.
- The continuous: I was hoping you could tell me the answer. Would it be possible for you to come back tomorrow?
- would: Would it be possible for you to come back tomorrow? Wouldn't it be better if you did it my way?
- Polite requests with can and could (possibly): Can I borrow your dictionary for a second? (Informal, not tentative) Could I borrow your umbrella? (Quite formal/polite) Could I possibly borrow your car on Saturday? (Formal/polite, very tentative)