Modal Verbs of Probability and Inference

English has a complex system of marking whether something was observed [It rained means that I saw it rain] or whether someone came to conclusion [It must have rained means that I’m nearly certain that it rained, because it is cloudy and the sidewalk is wet]. The degree of certainty can be modified by switching out must for may or could.

It is more natural to show this information through modal verbs than adverbs of probability such as certainly, probably or definitely. This can be seen by looking at the ngrams comparing probably rained, certainly rained and must have rained.

This is also why opinions aren’t explicitly marked by phrases such as in my opinion, to my mind or I think that often in English. A modal verb automatically tells the listener you are stating an opinion and your level of certainty. While the sentence In my opinion, it definitely rained isn’t a mistake, it sounds far more natural to say It must have rained.

The following story illustrates the context that various modals can be used in:

1) Let’s say you went to bed one evening in January and noticed that there was no snow on the ground.

Fantastic! There shouldn’t be any problems with traffic on the way to work tomorrow.

2) Unfortunately for you, when you woke the next morning you saw a thick covering of snow.

Uh-oh, it must have snowed last night

3) You decided to take the subway to work rather than driving.

The subway has to be running since it is underground. It must be unaffected by the bad weather.

4) When you got to the station and were about to pay for your fare, you realized that you didn’t have your wallet.

I might have left it at home or my wife may have accidentally put it in her purse.

5) It’s nearly impossible, but:

It could have been stolen.

6) Rather than worry about your wallet, you decided to sneak onto the subway without a ticket. When you finally got to the office, your friends are shocked and said:

You might have been caught. You could have been arrested.

7) One of your co-workers knows you better than the rest and realizes that you love jokes and exaggerating a bit.

You can’t be serious. You mustn’t be telling the truth since you have a reputation for telling tall tales!

Notes

1) should + [do], shouldn’t + [do]
Should or shouldn’t is used for a highly probable future event. The sentence He should be on time for the meeting. means that there is a high probability that he will be on time for the meeting rather than he ought to be on time for the meeting.

2) must have + [done], must not have + [done]
Must have shows that you have made a conclusion about a past event and are nearly certain. This can never mean a past obligation. For that we use had to. For example: Tony had to study for the exam because he wanted to get a good grade (obligation) vs. Tony must have studied for the exam because he got a good grade (inference based on evidence).

3) [have / has] to + [do], must + [do]
These equivalent structures show a conclusion about a present situation and need to be distinguished from obligation. Lisa must be at the office since she’s not home (inference) vs. Lisa must be at the office, because her boss won’t let her go home early. (obligation). In the present these have the same form but use different past forms: Lisa must have been at the office this morning since she wasn’t at home vs. Lisa had to stay at the office late, because her boss wouldn’t let her go home early.

4) might have [done], may have [done]
These structures have the same meaning: a conclusion has been reached about a past even with less certainty than must have.

5) could have [done]
This shows an even lower probability for a past even than might have.

6) could have [done], might have [done]
Might have and could have carry a different meaning than those in points four and five. In this meaning a possible past event that didn’t occur is described. It is important to distinguish this from point four since may have cannot be used here. For example, a mother has found out that her son was playing with matches and yells that he could have burned down the whole house and might have even killed himself.

7) can’t [do], mustn’t [do]
These structures show that a present situation is impossible. Note the difference between can’t / mustn’t showing an inference and the same words showing ability or prohibition. He can’t [must not] be in a good after he missed lunch. (inference) vs. He can’t [must not] run in the marathon due to his doctor’s orders (ability [prohibition]). The first example takes this form in the past: He couldn’t [must not] have been in a good mood after he had missed lunch. while the second would be He couldn’t [wasn’t allowed to] run in the marathon due to his doctor’s orders.

Additional Resources

  • See Murphy’s English Grammar in Use units 28 (must and can’t) and 29 (may and might) for additional practice.
  • If you are interested in a more in-depth discussion of tense, aspect and mood see the Wikipedia article on the topic.
  • Evidentiality is the formal term in linguistics for stating various levels of certainty.
  • Some languages in the Balkan Sprachbund have a completely separate verbal form to mark uncertain and reported information known as the inferential mood. This is more or less equivalent to the must have structure in English.

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