Modal verbs for deduction: must, might, and can't

Use deduction modals when you are working out what is probably true now. Start with a clue, such as a locked door, an empty desk, or a wet umbrella. Then choose a modal that matches the strength of your conclusion.

The floor is wet, so it must be raining outside.

The wet floor is the clue. Must be raining gives a strong conclusion from that clue.

You see a suitcase by the door and a taxi outside. What can you say about the visitor?

Say that the visitor must be leaving soon. The clues support a strong conclusion.

Use must when the clues make one explanation very likely. Must does not mean that you have direct proof. It tells the listener that the speaker has reached a confident conclusion. Put a base verb after must for every subject.

The bakery must be open because people are carrying fresh bread outside.

The customers and fresh bread make the conclusion about the bakery strong.

Choose a strong deduction: The screen is dark and the battery symbol is empty. The phone ___ out of power.

Use must be. The phone must be out of power. Be follows must in this state description.

Use might or could when an explanation is possible but uncertain. Both forms leave room for another explanation. In deductions about the present, might and could often have a similar meaning. Choose the form that sounds natural in the sentence, then keep the next verb in the base form.

The package might arrive this afternoon.

Might says that an afternoon delivery is possible, but the speaker cannot be sure.

The package could arrive tomorrow instead.

Could gives tomorrow as another possible outcome.

You hear music upstairs, but you do not know who is there. Make a possible deduction.

Say that they might be having a party. The music suggests a possibility, not a certain fact.

Use can't when the evidence makes a conclusion impossible. The form is can't plus a base verb. It is the negative opposite of must for deduction: must says a conclusion is very likely, while can't says it cannot be true.

That can't be Noor on the phone because she is sitting beside me.

Noor's presence beside the speaker rules out the caller's identity.

Choose the deduction: The shop is shut and the staff have gone home. It ___ open.

Use can't be. The shop cannot be open because the evidence makes that idea impossible.

Use modal plus be when your deduction describes a state, a place, or an identity. The word after be can be an adjective, a place phrase, or a noun phrase. This pattern is useful because many deductions describe where someone is or how they feel.

Mina must be tired after the overnight flight.

Tired is a state, so be comes between must and tired.

Your parcel might be at the reception desk.

At the reception desk gives a possible location after might be.

Complete the deduction about Amir being at home when the lights are on.

Use might be or must be, depending on how strong the other clues are.

To make a deduction about an action happening now, use modal plus be plus an -ing form. The modal still shows how certain you are; be having, be waiting, and be driving describe the action in progress. Do not use a past form after the modal.

Kai might be waiting for us at the entrance.

Might keeps the conclusion uncertain, and be waiting describes an action in progress.

The driver must be looking for a parking space.

Must gives a strong conclusion based on the driver's slow movement around the block.

Correct the sentence in which they must are looking for the address.

Write: They must be looking for the address. Use be, not are, after must.