Modal auxiliary verbs can express different degrees of certainty about an action or a fact.
The modal auxiliaries will, shall, can and must can be used to express complete certainty.
- She must have reached. (I am certain that she has reached.)
- You must be joking. (I am certain that you are joking.)
- There is the doorbell. That will be the postman. (I am certain that that is the postman.)
- That can’t be the postman. He has already been. (I am certain that that is not the postman.)
Probability and possibility
The modal auxiliaries should, may and ought (to) may be used to express probability or possibility.
- She should be here soon. (It is probable, not certain.)
- I may be taking a long leave.
- She ought to have reached.
To express weak probability, we can use might or could.
- He might win.
- You could be a millionaire one day.
Theoretical or habitual possibility
To express theoretical possibility we can use can.
- Glass can be blown.
- Mumbai can be very warm in April and May.
- If you had worked hard, you would have passed.
- If you invited, she would come.
- If you just stopped talking, I could get some work done.