These questions ask you to turn present evidence into a confident conclusion. Your task is to decide whether the clues strongly support an idea or rule it out, then build the modal phrase correctly.
Let the evidence choose the direction
Read the evidence before looking at the gap. If the clues point strongly towards one present state or activity, choose must. If they make an identity, state or activity impossible, choose cannot or can't. This is a logical contrast: positive certainty uses must, while negative certainty uses cannot or can't. Mustn't usually expresses a rule or prohibition, so it does not provide the ordinary negative counterpart for deduction.
| Signal in the sentence | Form to build |
|---|---|
| Strong clues support a present state or identity | must + be + complement |
| Strong clues support an activity happening now | must + be + -ing form |
| Strong clues rule out a present state or identity | cannot or can't + be + complement |
| Strong clues rule out an activity happening now | cannot or can't + be + -ing form |
Use a two-pass check. First state the conclusion without a modal and ask whether the evidence supports it or contradicts it. Then build the phrase: modal, bare be, and any complement. For an activity in progress, place the -ing form after be.
The traps
| Tempting form | How to reject it |
|---|---|
| Can't when every clue supports the conclusion | Can't reverses the direction of the evidence. When the clues strongly confirm the conclusion, use must. |
| Mustn't for a conclusion that the evidence rules out | Mustn't normally sounds like a ban. A confident negative deduction uses cannot or can't. |
| Must be not for negative deduction | Do not move not behind be to create the negative pattern. Choose cannot or can't before be. |
| Must to or can't to before the main phrase | A core modal is followed by the bare form, so use must be or can't be without to. |
Try it
Complete with a modal phrase for a confident present deduction and the verb in brackets: The water meter is moving although every tap is closed. A hidden pipe ___ somewhere. (leak)
Complete with the negative phrase formed from can and the adjective in brackets: The certificate claims to authenticate a Ming vase. The issuing authority's registry assigns its unique serial number to a washing machine and to no other object, so this certificate ___ for the vase. (authentic)
Complete with a modal phrase for a confident present deduction and the verb in brackets: Jo's account is streaming audio and her headphones have disappeared. She ___ them now. (use)