There are several auxiliary used to speak of degrees of probability. Without being exact numbers, we can say that: 90% Probability must
May/might - 50 %
Must not - 10 %
Can't / couldn't - 1%
As with other modal verbs is not "to" afterwards.
They must be chipper * after that long hike. They must be exhausted after the long March. [Almost certainly].
Diane might have the flu *.
It may have Diane influenza. [It is possible and is as likely as others].
Helen might not come to the family reunion.
It can Helen doesn't come to family reunification.
We use "must not", "may not" or "might not" to speak of a possibility which is contrary to the obvious or expected.
Scott must not do much exercise.
Scott should not do much exercise. [By his physique is unlikely to make sport].
Scott must not do much exercise.
Scott should not do much exercise. [By his physique is unlikely to make sport].
When we say that something is impossible, we use "can't" or "couldn't".
TYPE | USE | MODAL VERBS | EXAMPLE |
1 | to express possibilities for the future | MAY, MIGHT, COULD | It may rain tomorrow. |
2 | to express certainty | CAN'T, COULDN'T, MUST | She can't be Irish. |
3 | for possible explanations | MAY, MIGHT, COULD | She could be Scottish. |
TYPE | ||
1 | _______ | They (may/must)________________ be away for the weekend but I'm not sure. |
2 | _______ | He (can/could)________________ be French, judging by his accent. |
3 | _______ | They (can't/may)________________ still be out! |
4 | _______ | With luck, tomorrow (can't/could)________________ be a sunny day. |
5 | _______ | You (can/might)________________ be right but I'm going to check anyway. |
6 | _______ | The exam (can't/might)________________ be easy. You never know. |
7 | _______ | It (may/can't)________________ be true about a sauropod dinosaur living in Lake Telé in the Congo. It's impossible. |
8 | _______ | Dave reckons she's from The States but I think she (can't/might)________________ be from Scandinavia. |
9 | _______ | I (can't/might)________________ go to the party but I'm not sure yet. |
10 | _______ | This (must/could)________________ be the right answer but we'll have to check with your teacher to make sure. |
11 | _______ | She (can't/could)________________ steal things from shops. She's rich and famous. |
12 | _______ | I really think Real Madrid (can't/could)________________ lose the final of the King's Cup. |
13 | _______ | She's been revising 10 hours a day for 3 weeks. She (could/must)________________ be exhausted. |
14 | _______ | Nobody's answering. They (can't/must)________________ be out. |
15 | _______ | He (can't/may)________________ be from the USA. He doesn't speak English. |
