1. Overview:
Modal verbs of deduction are used to express different levels of certainty or possibility about a situation or a fact. They help us make assumptions based on the information we have. The main modal verbs for deduction are must, can't, might, may, and could.
2. Cases of Usage:
Case 1: Using "must" to express strong certainty about a present situation.
Example: She must be at home; the lights are on.
Case 2: Using "can't" to express strong certainty that something is not true.
Example: He can't be the thief; he was with me at the time.
Case 3: Using "might" or "may" to express a possibility or uncertainty about a present situation.
Example: She might come to the party later.
Example: They may be in the office now.
Case 4: Using "could" to express a possibility or suggestion that is less certain than "might" or "may."
Example: It could rain later, so take an umbrella.
3. Exceptions or Things to Pay Attention To:
When using "must," it often indicates a conclusion based on evidence, but it cannot be used for past situations.
Example: You must have seen the movie already (if someone tells you about it), but you can't say "You must see the movie last year."
In negative forms, "can't" and "must not" have different meanings. "Can't" indicates certainty that something is not true, while "must not" indicates prohibition.
Example: You can't drive without a license (you are not allowed to), but you must not drive too fast (it is prohibited).
4. Additional Notes:
Be careful not to confuse "might" and "may" as they are often interchangeable, but "may" is slightly more formal. Remember that "could" suggests a lower degree of certainty than "might" or "may."
