The sentence

2. An ‘uncertain’ question may produce a ‘certain’ answer:
Can he still be working? – Yes, he is.
3. We use be and have been to answer questions with be:
Is he ill? – He may be. Was he ill? – He may have been. (Not *he may*)
4. We use do/done to replace other verbs, though this is optional:
Will you catch the early train? – I may./I may do.
Has he received my message? – He could have./He could have done.
5. We also answer Wh-questions with ‘certainty’ or ‘uncertainty’:
What’s his name? – It’s Smith. (certain) It may/might be Smith. (uncertain)
What was his name? – It was Smith. (certain) It may/might have been Smith.
(uncertain)
E. Uses of modals of express deduction
Certainly or deduction?
1. We express certainty with be or any full verb: He is here. He lives here. He is leaving.
2. We express deduction with must be/can’t be, must have been, can’t/couldn’t have been:
He must be at home. He can’t be out. He must have been at home yesterday.
Or with verbs other than be: He must live abroad. He must have lived abroad.
Two kinds of ‘must be’
1. We use must be in the present to express deduction:
You haven’t eaten for hours. You must be hungry!
2. The negative of must be (= deduction) is can’t be, not *mustn’t be*:
You’ve only just eaten. You can’t be hungry again! (Not *mustn’t be*)
3. We can also use must (be) to express total obligation:
This is a hospital. You must be quiet.
4. The negative of must (be) (= total obligation) is mustn’t (be) (= prohibition):
You mustn’t be noisy outside a hospital.
‘Must have/been’, can’t/couldn’t have been’; ‘had to be/didn’t have to be’
1. We express deduction about the past with must have been:
You hadn’t eaten for hours. You must have been hungry!
2. The negative of must have been is can’t/couldn’t have been, not *mustn’t have been*:
You had already eaten. You can’t/couldn’t have been hungry! (Not *mustn’t have been*)
3. Must (= total obligation) is not a ‘complete verb’. We use had to in the past:
The meeting was at 10 this morning and I had to be there. (Not *must have been*)
4. The negative of had to is didn’t have to:
It was a holiday yesterday, so I didn’t have to be at work. (Not *mustn’t have been*)
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