These questions ask you to interpret evidence, regret or necessity after an event. First decide what the sentence means about the past; then build the required pattern with the correct auxiliary and verb form.
Decide the meaning before building the form
Begin with the evidence and the time reference. For a confident conclusion that something happened, use must have. If the evidence makes a past event impossible, use cannot have, can't have or couldn't have. For criticism, ask whether the advisable action was omitted or an unwise action was performed: should have points to what was not done, while shouldn't have looks back at what was done.
Necessity requires a separate happened-or-not check. Needn't have describes an action that really happened and later proved unnecessary. Didn't need to states that the action was unnecessary without implying completion; when the sentence explicitly says the person did nothing, this is the form that matches.
| Meaning signal | Pattern |
|---|---|
| Evidence strongly confirms a past event | must + have + past participle |
| Evidence rules out a past event | cannot, can't or couldn't + have + past participle |
| A desirable action did not happen | should + have + past participle |
| An unwise action happened | shouldn't + have + past participle |
| An action happened but was unnecessary | needn't + have + past participle |
| There was no necessity; completion is not implied | didn't need + to + bare form |
The traps
| Tempting form | How to reject it |
|---|---|
| A modal followed by had | After must, can't, should or needn't, the perfect auxiliary is always have, not had. |
| A base form or an -ing form after modal plus have | The final verb must be a past participle. Check irregular verbs as well as regular -ed forms. |
| Mustn't have for evidence that rules an event out | For confident negative deduction, use cannot have, can't have or couldn't have. Mustn't can suggest prohibition or criticism instead. |
| Shouldn't have when the advisable action was omitted | Shouldn't have criticises something that was done. If the better action never happened, use should have. |
| Needn't have when the sentence says the action was not done | Needn't have implies completion. If there was no necessity and the action did not happen, use didn't need to. |
Try it
Complete with a modal-perfect phrase for a confident past deduction and the verb in brackets: One suitcase is missing, and Dev's train ticket was scanned at dawn. He ___ the early train. (take)
Complete with the negative phrase formed from can and the verb in brackets: The vault alarm stayed armed all night, so the intruder ___ through its door. (enter)
Use a form with need and the verb in brackets to complete the sentence, describing a completed action that proved unnecessary: We ordered a translation headset for every guest and paid for them, but the talk was delivered in everyone's first language. We ___ so many. (order)