B2 First · Grammar · Pattern 6 of 10
Deduction and criticism about the past — must have done, can't have done, should have done, needn't have done.
1 of 8
I'm sure Jack didn't see us — he walked straight past.
HAVE
Jack us — he walked straight past.
A modal followed by 'have' and a past participle does two jobs at once: it says what the speaker thinks — certain, possible, wrong — and it puts the thought in the past. 'It must have rained' is a deduction from wet ground; 'we shouldn't have left the window open' is a criticism of something that already happened. Part 4 gives you the attitude in plain words — 'I'm sure', 'perhaps', 'it was a mistake' — and tests whether you can convert it into the modal.
The map is small. Certainty that it happened: must have. Certainty that it did not: can't or couldn't have. Possibility: might, may or could have. Criticism: should or ought to have. Unnecessary action that was done anyway: needn't have. Missed opportunity: could have. Six meanings, one structure.
The third form of the verb. 'Must have forgot' is the error the examiner sees most — the modal perfect needs the participle, and irregular verbs are where candidates slip. 'Forgotten', 'seen', 'been', 'brought'.
The negative deduction. 'Mustn't have' is not used for deduction in British English; it is 'can't have' or 'couldn't have'. And 'ought to' keeps its 'to' in the negative: 'ought not to have shouted', five words exactly.
Read the plain-English attitude in the first sentence and name it: sure, sure-not, maybe, wrong, unnecessary, possible-but-not-done. Each has one modal. Then 'have', then the participle. The key word usually names the modal or is HAVE; either way the structure does not change.
'Needn't have done' and 'didn't need to do' are the one pair with a real difference: the first says you did it and it was pointless, the second usually says you did not do it. When the first sentence makes it clear the thing was done, 'needn't have' is the precise answer.
'Might have' is a guess about what happened ('he might have missed the bus'). 'Could have' can be the same guess, but more often in Part 4 it means a possibility that was not taken ('you could have asked for help'). The first sentence tells you which — 'perhaps' points to might, 'why didn't you' points to could.
Yes in meaning — both criticise a past action or its absence. 'Ought to' is more formal and keeps its 'to' in every form, which is why the exam uses it: 'ought not to have shouted' tests whether you know that 'to' stays.
