Modals of Deduction (Past)

• When we look at past evidence in a clinic or hospital in Boston or Seattle, we use past modals of deduction to make logical guesses about what happened.
• Meaning
• Modal
• Example
• Almost certain (Yes)
• must have + past participle
• She must have eaten contaminated shellfish.
• Possible
• might have + past participle
• He might have inhaled mold spores at work.
• Almost certain (No)
• can't have / couldn't have + past participle
• They can't have been exposed to carbon monoxide.
• Remember
• Use 'must have' when you are almost sure something happened in the past.
• Use 'might have' when you think it is possible that something happened.
• Use 'can't have' or 'couldn't have' when you are almost sure something did not happen.
• Always use the past participle (eaten, inhaled, been) after have.
• Examples
• The patient must have ingested the poison yesterday.
• Dr. Miller thinks she might have had an allergic reaction.
• He couldn't have contracted lead poisoning at his new office in Chicago.
• Common mistakes
• She must have took ibuprofen.
• She must have taken ibuprofen.
• They couldn't has been exposed to fumes.
• They couldn't have been exposed to fumes.

Complete the sentences

• The lab results are clear; the patient (must / ingest) ____ the undercooked food in New York.
• Dr. Davis said she (might / be) ____ exposed to mold in her basement.
• They (can't / eat) ____ the shellfish because they hate seafood.
• He had no rash, so he (couldn't / have) ____ a bee sting.
• The nurse looks tired; she (must / work) ____ a double shift at Boston General Hospital.
• The tests were negative, so she (might / catch) ____ a different virus.

Sources: photo by Ramesh Kambattan on Pexels