Modals of Deduction (Present)

• We use modal verbs of deduction to make logical guesses about present situations based on available evidence. The choice of modal depends on how sure we are about our conclusion.
• Modal
• Degree of Certainty
• Example
• must
• 95% sure (logical conclusion)
• Her blood test results are positive; she must have an autoimmune disease.
• might / could
• 50% sure (possibility)
• The patient's cough might be tuberculosis.
• can't
• 95% sure (impossible)
• His lung scan is completely clear; this can't be pneumonia.
• Remember
• Use must when the evidence strongly supports a conclusion.
• Use might or could when something is possible but you need more tests.
• Use can't when the evidence makes a diagnosis impossible.
• Always use the base form of the verb after must, might, could, and can't.
• Examples
• Dr. Miller looks tired; he must work too many night shifts at Boston General Hospital.
• This skin rash could be lupus, but we need an ANA test to confirm.
• The patient's temperature is normal, so this can't be a severe infection.
• Common mistakes
• The diagnosis must to be correct.
• The diagnosis must be correct.
• She can't has lymphoma.
• She can't have lymphoma.

Complete the sentences

• The patient's swollen joints and butterfly rash mean she (must) ____ have lupus.
• All his lung tests are completely normal, so this (can't) ____ be sarcoidosis.
• His persistent cough and night sweats suggest it (might) ____ be tuberculosis.
• Given his neck swelling and fatigue, the problem (could) ____ be a thyroid issue.
• With those swollen lymph nodes and persistent fever, it (might) ____ be lymphoma.
• Since the antibody tests are entirely negative, it (can't) ____ be an autoimmune condition.

Sources: photo by Mary Taylor on Pexels