Deduction Modals Exercises

Modal verbs for deduction allow you to make logical conclusions based on evidence. When you are almost certain something is true, use must: "She must be at home — her car is in the drive" (I am 95% sure based on evidence). When you believe something is impossible, use can't or couldn't: "He can't be French — he doesn't speak a word of French" (I am 95% sure it is impossible). For things that are possible but uncertain, use might, may, or could: "She might be ill — she didn't come to work today" (about 50% sure).

For past deductions, add "have" + past participle after the modal: "He must have left early" (I am almost sure he did), "They can't have seen us" (it was impossible), "She might have taken the wrong train" (it is possible). The level of certainty goes: "must" (strongest positive) → "may/might/could" (possible) → "can't/couldn't" (strongest negative). A common mistake is using "mustn't" for negative deduction — mustn't means prohibition (you are not allowed), not impossibility. For negative deduction, always use "can't" or "couldn't": "It can't be true" (not "It mustn't be true"). This distinction is heavily tested in Cambridge B2 First exams.