oxford_guide_to_english_grammar

PAGE 121
96 Probability
NOTE
a In informal English we can sometimes use have (got) to for logical necessity.
There has to/has got to be some mistake.
b We can also use be sure/bound to.
Carl is sure to/is bound to be sitting in a cafe somewhere.
c For can't and mustn't in the USA, • 303(10).
2 In questions we normally use can or will.
Who will/can that be at the door? Can it really be true?
But can for possibility has a limited use in statements. • 97(2e)
3 We can use the continuous or the perfect after will, must and can't.
Where's Carl?~ He'll be sitting in a cafe somewhere, I expect.
The bus is ten minutes late. It must be coming soon.
This glass is cracked. Someone must have dropped it.
I can't have gone to the wrong house. I checked the address.
Compare must have done expressing certainty about the past and had to
expressing a past necessity.
This film seems very familiar. I must have seen it before.
Everyone had been telling me about the film. I had to see it.
But for another meaning of had to, • (5).
4 Must do is usually a kind of order, a way of telling someone to do something. Must
be doing usually means it is logically necessary that something is happening.
You've got exams soon. You must work. (order)
Paul isn't at home. He must be working. (logical necessity)
5 We can use would, had to and couldn't when something seemed certain in the past.
There was someone at the door. It would be the milkman.
The fingerprints were the husband's, so he had to be the murderer.
Harold stared in amazement. It couldn't be true!
96 Probability: should and ought to
We use should and ought to to say that something is probable, either in the present
or the future.
They should have/ought to have our letter by now.
We should know/ought to know the result soon.
In the negative the usual form is shouldn't.
We shouldn't have long to wait.
Should and ought to have the additional meaning 'if all goes well'. We cannot use
these verbs for things going wrong.
The train should be on time. but NOT The train should be late.
NOTE
To express probability we can also use be likely to or will probably.
We're likely to know the result soon./We'll probably know the result soon.