Advanced Modals: Using Modal Verbs of Probability: deduction and speculation

Modal verbs of probability, also referred to as modals of deduction, speculation, or certainty, are used to express the likelihood that something may or may not be true. We can use modals of probability to speculate on what is happening in the present, or on what has happened in the past.

Using Modals of Probability to speculate on the present

We often wonder what is happening to cause a present situation to exist. One way we express this curiosity is to use the verbs, must, might, could, may, or can’t, followed by an infinitive.

  • must, might, could, may, or can’t, PLUS infinitive

These verbs express a range of certainty, with “must” and “can’t” used for solid guesses, and the rest falling in the middle as possibilities.


For example, imagine that you wonder why it seems so hot in your apartment. Here are some speculative statements about that:

  • The air conditioning must be on.
  • The air conditioner might need more time to work.
  • We could be experiencing a blackout.
  • It may be too hot for this air conditioner to work.
  • I can’t be imagining that it’s too hot.

If we feel especially certain about what’s happening and don’t believe we’re making a guess, we use will and won’t.

  • These tiny wall units won’t cool an apartment this big.

We use “should” and “shouldn’t” if we’re sure something is the case so long as everything else is behaving as expected.

  • This air conditioner should work.
  • This air conditioner shouldn’t be broken already.

However, if we’re sure of a negative even based on expected circumstances, we use “will” instead of “should.”

  • The heat time of year will cause more power outages than usual.

Finally, we use “can” to speculate based on something that happens generally.

  • The temperature can be high this time year.

The exception to this rule is that we don’t use “can” to speculate based on specifics.

  • “The temperature could be 40 degrees ,” not “The temperature can be 40 degrees.”

Speculating on the Past with modal verbs

We can also use modal verbs to speculate on what might have happened in the past. We generally do this by adding “have” and a past participle to the modal verb.

For uncertain guesses, we would use the following formula:

(must, or might/might not, or could/couldn’t, or may/may not, or can’t) + have + past participle

For example, let’s say you can’t find the leftover cheeseburger from last night’s meal in the refrigerator. Here are some speculative statements about why it is not there:

  • I must have left the cheeseburger on the counter.
  • The cat might have eaten
  • I could have forgotten it at the restaurant.
  • My wife may have taken it to work.
  • I can’t have finished it in my sleep.

When we’re more certain about what happened in the past, we use (will, or won’t/will not) + have + past participle:

  • He won’t have left home before six.

We can use should/shouldn’t + have + past participle when guessing about the past based on standard expectations:

  • He should have arrived at the office by now.

Speculations on the past in general are constructed similarly to speculations on the present in general, except that we use could + an infinitive instead of “can” (see the example above):

  • The Victorian age could be difficult for some people.

However, when speculating more specifically on the past, we use could + have + past participle:

  • “She could have been at the movies last night,” not “She could be at the movies last night.”