Advanced English Grammar

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 advanced level.

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 will.

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 (to).
We should arrive by noon.
We ought 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 past participle.
He should 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 didn't.
Yesterday should 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 something.
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

1. They ____ have been here by now. What's the time?
2. She ____ be at work. Today's her day off.
3. That ____ be kole at the door, he said he'd stop by.
4. You haven't eaten today? You ____ be starving!
5. The weather forecast for tomorrow is excellent. Yes, it ____ be lovely.
6. Your son got a scholarship? You ____ be very proud.
7. She ___ have failed the exam, she knew all the answers.
8. He opened a business in the new mall. I know it ____ do well.
9. He ____ be home by now. He just left.
10. "My coat isn't where I left it." "Someone ____ have taken it."

For more information concerning the modal verbs of probability click on the preceding link.

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