Grammar B2 - Modals of Deductions

The video explains how to use modal verbs for deduction, comparing the process to a detective investigating a mystery based on evidence. The speaker emphasizes that the choice of verb reflects the speaker's level of confidence in their theory.

Deducing the Present

When observing a situation in the present, different modal verbs are used based on the level of certainty:

• Must: Used when you are about 95% sure something is true. For example, if a TV is playing loudly, you can deduce that someone "must be home".
• Can't: Used to express a strong deduction that something is impossible. For example, if a house is dark and a car is gone, you can deduce that someone "can't be home".
• Might/Could: Used when you are unsure and just guessing about a possibility. For example, if a light is on but no car is present, you might say someone "might be home" or "could be home".

Deducing the Past

To make deductions about the past, the structure of the sentence changes by adding "have" plus the past participle of the verb. The speaker describes "have" as a "time machine" that shifts the guess into the past.

• Must have: Used for a 95% certain deduction about a past event. If the ground is wet, you conclude it "must have rained".
• Can't have: Used when it is logically impossible for a past event to have occurred. For example, if someone failed an exam after partying all night, you would say they "can't have studied".
• Might have/Could have: Used for uncertain guesses about the past. For example, you might say someone "might have studied" but the test was simply difficult.

The speaker highlights that the most common error is omitting "have" when discussing the past, which results in accidentally making a guess about the present instead.