C1 · Lesson 3 · 25 min
Probability and degrees of certainty
In this lesson
- I can distinguish deductions from obligations and possibilities.
- I can use perfect and continuous modal forms to make the time of a deduction clear.
Grammar in context
The studio lights are on, and I can hear the printer. Jo must be working late. She may have received an urgent order. The courier is likely to arrive before the studio closes, but we cannot be certain.
Which statement is a strong deduction, which describes a possible earlier event, and which predicts a likely future event?
Match strength to evidence
Must can express a strong deduction based on evidence rather than an obligation. May, might and could allow a possibility without presenting it as certain. Cannot or can’t can express a strong negative deduction when the evidence rules something out. These choices communicate the speaker’s confidence; they do not independently prove what happened. In ordinary deductions, do not use must not as the automatic negative equivalent of must: it often expresses a prohibition instead.
- The car is gone; she must be out.
- That cannot be our bus; its destination is different.
Show the time and duration
For a deduction about an earlier event, use a modal plus have and a past participle: must have left. For a possible activity in progress now, use a modal plus be and an -ing form. Have been plus an -ing form places the continuing activity before the current reference point. Select the form from the meaning you intend, and keep past obligation separate: had to leave reports a necessity, whereas must have left normally infers an earlier departure.
- They might be discussing the schedule now.
- He must have been waiting for some time; his coffee is cold.
Use likely and bound with be
Be likely to and be unlikely to describe a probable or improbable event. Be bound to usually communicates a strong expectation, although context still matters and the event is not logically guaranteed. Inflect be for the subject and time, and follow to with a base verb. A that-clause is another option after impersonal it is likely. Choose language proportionate to the evidence instead of presenting a cautious prediction as an established fact.
- The revised timetable is likely to cause delays.
- With so many applications, the panel is bound to need more time.
Common mistakes
Compare the incorrect and corrected forms.
Avoid
She must left before we arrived.
Use
She must have left before we arrived.
A deduction about an earlier departure needs must have plus the past participle.
Avoid
The package likely to arrive tomorrow.
Use
The package is likely to arrive tomorrow.
This likely to construction needs a form of be.
Put it into practice
Try the exercises, write your own examples and check your understanding.