The sentence

- present form: He must be right. He must know the answer. (now)
- perfect or past form: He must have been right. He must have known the answer.
(then)
B. Uses of modals (etc> to express ability and inability
Expressing present and past ability: ‘ can’ and ‘be able to’
1. We can use can (or sometimes am/is/are able to) to describe natural or learned ability:
I can (I am able to) run 1500 metres in 5 minutes. (natural ability)
I can’t (I am not able to/I am unable to) drive. (learned ability)
2. We can use could, couldn’t or was/were (not) able to to describe ‘general ability in the
past’:
3. We use was/were able to or managed to (Not *could*) to describe the successful
completion of a specific action:
We were able to (we managed to) get ticket for the match yesterday. (Not *could*)
4. However, we can use couldn’t to describe a specific action not successfully completed:
We couldn’t get tickets for the match yesterday. Or:
We weren’t able to/didn’t manage to get tickets for the match yesterday.
‘Can/could’ with verbs of perception: ‘I can see’
1. Verbs of perception are verbs like see, hear, smell, etc.
2. When we are describing something that is happening now, we do not use the progressive
with these verbs: I see a bird in that tree. (Not *I’m seeing*)
3. We often use can + verb in place of the simple present with verbs of perception:
I can see a bird in that tree. (= I see) Can you see it? (= Do you see)
We often use could + verb in place of the simple past with verbs of perception:
I looked up, but couldn’t see anything. (= didn’t see)
Ability in tenses other than the present and the past
Can and could are not ‘complete verbs’, so we use be able to and sometimes manage to if,
for example, we want to express the future or the present:
I’ll be able to pass my driving test after I have had a few lessons. Not *I can/I will can*
‘Can/could’ in place of ‘is often’ and ‘was often’
Instead of: It’s often cold in January. We can say: It can be cold in January.
Instead of: He was often naughty when he was a boy.
We can say: He could be naughty when he was a boy.
C. Uses of modals (etc.) to express permission and prohibition
Asking for permission with ‘can’, ‘could’, ‘may’ and ‘might’
We use can, could, may and might to ask for permission, depending on the situation.
1. Can is the commonest and most informal: Can I borrow your umbrella (please)?
2. Could is more polite than can: Could I borrow your umbrella (please)?
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