Collins Cobuild

Unit 82
Probability and certainty
Main points
You use ‘must’, ‘ought’, ‘should’, or ‘will’ to express probability or
certainty.
You  use  ‘cannot’  or  ‘can’t’  as  the  negative  of  ‘must’,  rather  than
‘must not’ or ‘mustn’t’, to say that something is not probable or is
not certain.
1
When you want to say that something is probably true or that it will
probably happen, you use ‘should’ or ‘ought’. ‘Should’ is followed by
the base form of a verb. ‘Ought’ is followed by a ‘to’-infinitive.
We should arrive by dinner time.
She ought to know.
When you want to say that you think something is probably not true
or that it will probably not happen, you use ‘should not’ or ‘ought not’.
There shouldn’t be any problem.
That ought not to be too difficult.
2
 When  you  want  to  say  that  you  are  fairly  sure  that  something  has
happened, you use ‘should have’ or ‘ought to have’, followed by a past