Present Deduction with must, might and can’t

What is a present deduction? A deduction is a conclusion you reach by using the information or evidence available to you. You do not know something as a direct fact, but the situation gives you clues. For example, if Maria’s car is outside her house and the lights are on, you can make a deduction about where she is. English commonly uses must, might and can’t to show how certain you are about deductions like this. Use must when you are very sure Use must when the evidence gives you a strong reason to believe that something is true. In She must be tired, the speaker is not giving an order. The speaker has some evidence, such as knowing that she worked all night, and concludes that being tired is very likely. Use might when something is possible Use might when you think something is possible but do not have enough evidence to be confident. There may be several possible explanations. If Ben is not at his desk, you could say He might be at lunch. You are suggesting one reasonable possibility, not stating a fact. Use can’t when you are very sure something is not true Use can’t when the available evidence strongly suggests that something is impossible or extremely unlikely to be true. If you know that Alex is currently in Madrid, you can say That can’t be Alex when you see someone who looks like him in London. Here can’t expresses deduction, not inability. Think about the strength of the evidence The main difference between these modals is the speaker’s level of certainty. Strong evidence that something is true usually leads to must. Limited evidence or uncertainty leads to might. Strong evidence that something is not true leads to can’t. The same situation can sometimes produce different deductions depending on how much information the speaker has. The verb form after a modal After must, might and can’t, use the base form of the verb without to: must know, might need, can’t understand. When your deduction is about a state, identity, location or characteristic, you will often use be: must be tired, might be at home, can’t be serious. Modal verbs do not take an -s ending with he, she or it. What about negative and question forms? For a possible negative situation, use might not: She might not know about the change. For a strong negative deduction, use can’t rather than mustn’t: He can’t be at work; the office is closed. Direct deduction questions with must or might are less common. In everyday English, speakers often ask neutral questions such as Could he be at home? and then answer with a deduction such as He might be.