Modal Verbs of Probability
Modal verbs of probability are often tricky for non native speakers of the
English Language for a number of reasons. First, there is the notion that these words somehow express a percentage,
or degree, of probability.
I've seen a number of grammar books over the years explain these modals using a
'percentage of certainty' -- saying that if you are 30% sure something is true then use should, 60% ought to, 90%
and above, use must. (for example)
A quick illustration demonstrates that this line of thinking is erroneous, although
for some practical purposes in teaching this subject, I can understand the thinking behind it. After all, teachers
are trying to get their students to use the language according to rules.
While rules are good and necessary, a feel for the language needs to be developed at the
For instance, there is no percentage of certainty that something is, or isn't, given
the use of a certain modal verb. If we say "It must be him," "it ought to be him," and "it should be him," -
depending on the situation - we are, in fact, saying the exact same
thing. To the mind of the native speaker, we are certain that
it IS him.
The modal verbs of
probability are cannot, (can't,) must, ought, should, and
Must, ought, should,
will, are used to express certainty. Should or ought express less certainty. Should is
followed by the base form of a verb, or bare infinitive, while ought is followed by the full infinitive
We should arrive by noon.
to arrive by noon.
We will arrive by noon.
When you say that you are fairly certain that something
has happened, we use
should have or
ought to have followed by a
have heard from them by now, it's been a week.
They ought to have
arrived by now, their plane landed two hours ago.
We use should
have or ought to
have to say that you expected something to happen -- but it
have been the start of the basketball season.
She ought to
have been made manager by now.
We use must to show that we are fairly certain about
Hello, you must be John's wife.
We do NOT use mustn't in the same way, we use can't, or cannot.
Hello, you can't be John's wife. NOT MUSTN'T
Modal Verbs of Probability Exercise
For more information concerning the modal verbs of probability click on
the preceding link.