Imagine your colleague, David, is usually at his desk by 9 a.m. sharp. It’s now 9:30 a.m. and his chair is empty. You don’t know for sure where he is, but you can make some logical guesses, or deductions, based on what you know.

In English, we use modal verbs to show how certain we are about these deductions. For situations happening now, the structure is simple: modal verb + be / verb.

For example, if you know David takes the train, you might think:

• "There are delays on the train line this morning. He must be stuck."

Here, must shows you are very certain. You have evidence (the train delays) that leads you to a logical conclusion. What if you have no evidence?

• "He might be sick." or "He could be working from home today."

Might and could express possibility. You're not sure, but it's a reasonable guess.

Now, what if someone suggests he's on holiday?

• "He can't be on holiday. He told me yesterday he had a big deadline today."

Here, can't expresses a strong belief that something is impossible.

Making deductions about the past is just as common. Let’s go back to our colleague, David. The next day, you learn he missed a critical meeting yesterday. You don't know why, so you speculate about what happened.

To do this, we use a different structure. This structure is essential for discussing past possibilities.

For example:

• Strong Deduction: You hear David's car broke down. You can say, "He must have had car trouble." You have evidence, so you're making a strong, logical conclusion about the past.

• Possibility: If you have no information, you can only guess at the possibilities. "He might have been sick," or "He could have overslept."

• Impossibility: What if you saw David looking perfectly healthy at 5 p.m. yesterday? "He can't have been sick all day. I saw him in the evening."

Notice that for past possibility, may have is also used, but it's slightly more formal than might have or could have.

There's another common past modal form that looks similar but has a completely different meaning. It's not about deduction; it's about criticism or regret.

We use should have + past participle to talk about a better action or choice in the past. It’s like giving advice to someone about something that already happened, or expressing your own regret.

• Criticism of others: "He missed the deadline. He should have started his work earlier."
• Regret about your own actions: "I failed the exam. I should have studied harder."

This structure implies that the right thing did not happen. A similar phrase is ought to have + past participle. It means the same thing as should have but is a bit more formal and less common in casual conversation.

• "The company is losing money. They ought to have listened to their customers."

Crucially, should have is not used for guessing. It's for judging a past situation with the benefit of hindsight.

Ready to test your skills? Let's see how well you can use these modal verbs.

Using modals for deduction and regret is a key step towards sounding more natural and nuanced in English. It allows you to express not just what happened, but what you think about what happened.