We can use modal verbs to show our level of certainty; the modals of certainty are divide into two parts: certain and uncertain
Modal verbs can be used to express ideas about the past, present, and future. The most definite degree of certainty can be expressed with “will” and won’t, followed by must and can’t/couldn’t, should and shouldn’t, may and may not, and might/could and might not, which express the least definite degree of certainty.
Present
A person who is 100 percent sure uses the verb be, as in, “I am sick.” or Heisn’thungry.
If they are mostly sure, say 95 percent, they will use the modal must, as in:
- “I must be sick.”
- “I must call.”
- “Sam can’t be hungry.”
- “Sam must not be hungry, he didn’t eat dinner with us.”
When speakers are about 50 percent or less sure, they will use the modals may as in
- “I may be sick.”
- “She may be in class.”
- “Tina may not be in her house.”
Past
If we are sure that something is true in the past, we don’t need to use a modal we just use the verb to be in its past form:
- She was sick.
- He was in Germany.
- Clint and Jonh were not interested.
If they are mostly sure, say 95 percent you would say:
- “She must have been sick.”
- “They might have been in the office.”
- “George could have not been with Gena.”
When speakers are about 50 percent or less sure:
- She may have been sick.
- They may have not been at the party.
- She could have been sick.
- Linda and Franklyn could have not been to Spain.
Future
When you are sure 100%:
- He will do well on the test.
- I am going to phone Elenor later.
- Jill and her kids will be here on time.
- I am not going to write that later.
- I will not be there next week.
If you are mostly sure, say 95% you would say:
- He should do well on the test.
- He should not be in the lab at nine.
- Mary ought to do well on the quiz.
- Louis ought to call at five.
When you are about 50% or less sure:
- She may do well on the test.
- We may not be there for him next month.
- He might do well on the test.
- Jean might lose her temper after you give her the news.
- He could do well on the test.
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