Advanced modality
Express degrees of certainty, reproach, unrealised plans and formal arrangements with precise modal forms.
25 minProficiency
1The grammar
Modal perfects express judgements about the past. 'Must have' shows confident deduction, 'can't have' or 'couldn't have' shows that something seems impossible, and 'may/might/could have' shows possibility. 'Should have' and 'ought to have' express unfulfilled obligation or expectation. 'Might have' and 'could have' can also express reproach: 'You might have warned me.'
'Needn't have done' means that an action happened but was unnecessary: 'We needn't have booked; the restaurant was half empty.' 'Didn't need to' only says it was not necessary, and it doesn't tell us whether the action took place. 'Needn't have' is more common in British than in American English, which often uses 'didn't need to' or 'didn't have to'.
'Be to' expresses formal arrangements or instructions: 'The minister is to visit Nairobi.' In the past, 'was to + base verb' can report a later fate ('He was to become president'), while 'was to have + past participle' shows a plan that did not happen ('She was to have spoken, but she fell ill').
'Will' can express a confident assumption about the present: 'That'll be the courier.' After adjectives of surprise or judgement, 'should' can present an idea without asserting it as a simple fact: 'It's strange that she should say that.' 'May well' and 'might well' mean that something is quite likely, and 'might as well' suggests that there is no better option.
2Structure and formation
Deduction about the past
must / can't / couldn't / might + have + past participle
They can't have received the invoice.
Unnecessary past action
needn't + have + past participle
You needn't have translated the annex.
Reproach
might / could / should + have + past participle
You could have told us about the audit.
Formal plan or later fate
be + to + base verb
The delegation is to arrive on Monday.
Unrealised plan
was/were + to have + past participle
The merger was to have been completed in June.
3Examples
The package must have been delivered to the wrong address.
A strong deduction about the past, in the passive.
They can't have received the invoice, or they would have paid.
The speaker considers it impossible.
We needn't have booked; the restaurant was half empty.
They booked, but it was not necessary.
You might have warned me that the inspectors were coming.
This is a reproach, not a statement of possibility.
The president is to visit Nairobi next month.
'Be to' reports an official arrangement.
The summit was to have taken place in Oslo, but it was cancelled.
The plan did not happen.
That will be the finance director; she said she'd ring at ten.
'Will' expresses a confident assumption about the present.
4Common mistakes
The meeting must be finished hours ago.
The meeting must have finished hours ago.
A deduction about a past event needs must + have + past participle.
It may rain yesterday, which would explain why the roads are wet.
It may have rained yesterday, which would explain why the roads are wet.
Possibility in the past needs a modal perfect.
We needn't booked the tickets.
We needn't have booked the tickets.
The modal perfect requires 'have' before the past participle.
She must to have left early.
She must have left early.
'Must' is followed by a bare infinitive, without 'to'.
5Practical usage
News and official language
'Be to' is typical in headlines and official notices, such as 'PM to visit Kenya' and 'Staff are to report any incident to their manager.'
Hedging and tone
Modal perfects let you judge the past with an appropriate degree of certainty. They can sound accusing, so choose 'might have' or 'should have' with care in professional messages.
Now practise it
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