Modals of deduction (present): must, might, could, can't

• We use modal verbs of deduction to make guesses about situations in the present based on evidence. The choice of modal depends on how sure we are.
• Modal
• Degree of certainty
• Example
• must
• Almost 100% sure (logical conclusion)
• The server must be down.
• might / could
• Possible (around 50% sure)
• The payment step might have a bug.
• can't
• Almost 100% sure it is impossible
• Users can't like this new layout.
• Remember
• Use must when the evidence strongly supports your conclusion.
• Use might or could when you think something is possible but you are not certain.
• Use can't when you are almost certain something is impossible.
• Do not use mustn't for negative deduction; always use can't.
• Examples
• Look at that massive spike in traffic; it must be a bot attack from Chicago.
• The designer could be working on the mobile version in the Seattle office right now.
• That error message can't be correct because we tested it yesterday in Austin.
• Common mistakes
• The conversion rate must to be low.
• The conversion rate must be low.
• This button mustn't work.
• This button can't work.

Complete the sentences

• Traffic is completely flat on the dashboard, so visitors (not be) ____ seeing the new features in Boston.
• The analytics show a sudden drop at checkout, which (be) ____ caused by the recent code release in New York.
• That strange dip in sales (might) ____ relate to the server issues in San Francisco.
• The checkout funnel is failing completely, so users (not finish) ____ their purchases in London today.
• Look at those abandoned carts; there (must) ____ be a problem with the payment step in Seattle.
• We are not sure why, but the bounce rate (could) ____ increase further tomorrow in Boston.

Sources: photo by Ramesh Kambattan on Pexels