Modals of deduction like must, can’t, and might combine with the perfect continuous infinitive, have been plus -ing, to guess about actions ongoing at a past point, as in he must have been sleeping when the alarm went off. Weaker deductions use might, may, or could have been.
Grammar Rule
Modals of deduction (must, can’t/couldn’t, might, may, could) combine with the perfect continuous infinitive (have been + -ing) to express guesses about actions ongoing at a past point. Weaker deductions use might/may/could have been. The continuous form is chosen over the simple perfect infinitive when duration or an ongoing process is emphasised, rather than mere completion.
Modals of Deduction in Continuous and Perfect Continuous Forms: Rules and Examples
| Form | Certainty | Example |
|---|---|---|
| Must have been + -ing | near-certainty | He must have been sleeping when the alarm went off. |
| Can’t/couldn’t have been + -ing | confident disbelief | She can’t have been working all night. |
| Might/may/could have been + -ing | weaker deduction | They might have been waiting for hours. |
Remember: Use modal + have been + -ing to deduce that a past action was in progress or ongoing; stative verbs generally resist this form.
Practice Test
Instructions
Answer each question in the test below using modals of deduction with the perfect continuous correctly. You will get a new random set of questions every time you retake the test, plus a downloadable results summary with your name and score at the end.
C1-30 Modals of Deduction in Continuous and Perfect Continuous Forms
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Frequently Asked Questions
What does must have been plus -ing express?
A near-certain deduction about a past ongoing action, as in he must have been sleeping when the alarm went off.
What does can’t/couldn’t have been plus -ing express?
Confident disbelief, as in she can’t have been working all night.
What do might/may/could have been plus -ing express?
A weaker deduction, as in they might have been waiting for hours.
When is the continuous form chosen over the simple perfect infinitive?
When duration or an ongoing process is emphasised, rather than mere completion.