Modal Verbs of Probability for English-Language Learners

These verbs express an opinion held by the speaker

Types of Verbs
Kenneth Beare

Modal verbs of probability are used to express an opinion of the speaker based on information that the speaker has. Put another way, you use modal verbs when you want to guess something, notes Perfect English. For example, "He must be at work; it's 10 o'clock." In this sentence, the speaker is nearly sure that the person is at work based on the speaker's knowledge that the person in question usually works during the day.

But the speaker does not know for sure, which makes the verb must a modal verb of probability.

When finished reviewing the various modal verbs, take the quiz—or have students take the quiz—after the examples. For ease of grading, the answers follow the brief test.

Using Must in Present and Past Tense

Use must plus the verb when you are almost 100 percent sure that something is the case. The construction would be:

  • Present = must + verb (do)

Some examples of the modal verb must in the present tense include:

  • They must be in Spain by now. They told me they were going last week.
  • Jack must think I'm crazy because I think grammar is easy!

The construction for the modal verb must  in the past tense is:

Examples of the modal verb must in the past tense include:

  • Anna is smiling. She must have done well on the test.
  • Alice must have asked for some help on the test because she got an A.

    Using Might or May

    Use might or may to express an opinion that you think has a good possibility of being true. The construction would be:

    • Present = might / may + verb (do)

    Examples of using might or may in the present tense include:

    • She might come this evening, but she also had some work to do.
    • David may invite Jessica to the match. I know he really likes her.

      The construction for may and might in the past tense is:

      • Past = might / may + have + past participle (done)

      To use might as a modal verb in the past tense, you could say:

      • Jack might have gone to France for her vacation. I think he wanted to practice French this summer. 

      Using Could

      Use could to express a possibility which is one of many. This form is not as strong as might or may. It is just one of a number of possibilities. The construction in the present would be:

      • Present = could + verb (do)

      Examples of using could in the present tense in dialogue include:

      • Jane could be at work, or she could be at home. I'm not sure.
      • We could hire that company or the other. It doesn't really matter.

      The construction of could in the present tense is:

      • Past = could have + past participle (done)

      Examples of the modal verb could in the past tense include:

      • Peter could have arrived late. I know he missed the bus.
      • Alice was tired. She could have stayed at home today, or she might have gone to work. 

      Can't or Couldn't 

      Use can't to express an opinion that you are 100 percent sure is not true. Use must be or must have been if you are sure in a positive sense but can't be, can't have been, or couldn't have been if you are sure in a negative sense.

      Note that the past form is couldn't have done. The construction for can't  in the present tense is:

      • Present = can't + verb (do)

      Examples of this modal verb as used in dialogue include:

      • You can't be serious! I'm not going to loan you $1 million dollars!
      • Peter can't like that show. He doesn't enjoy comedy.

      The construction of can't or couldn't in the past tense is:

      • Past = can't / couldn't + have + past participle (done)

      Examples of can't and couldn't as used in dialogue include:

      • They can't have worked until late because they were on time for the meeting.
      • She couldn't have believed that story. She knows he's a liar!

      Modal Verbs of Probability Quiz

      Use must, might, may, could, or can't plus the correct form of the verb to fill in the blanks in the quiz. In some cases, there is more than one right answer.

      Pay close to attention to time words to conjugate the modal verb of probability correctly. 

      1. Where is David? He __________ (be) at school. Classes begin at 8 a.m., and he's never late.
      2. She __________ (think) that it is a good idea. It's crazy!
      3. I'm absolutely sure! They __________ (arrive) yesterday. Tom showed me his train ticket.
      4. Courses __________ (begin) on Sept. 5, but I'm not really sure.
      5. Are you joking! David __________ (go) to Paris last week. He doesn't have enough money to go to Europe. 
      6. They __________ (live) in New York, or he __________ (be) in San Francisco. I know he likes big cities.
      7. The concert __________ (be) wonderful last night. John is a fantastic singer. You __________ (have) fun.
      8. Students __________ (get) sick and tired of grammar. I know it's kind of boring. 
      9. Alice __________ (be) looking for a job because she finished college last year.
      10. Janice __________ (want) to get in touch with you. She's always asking about you when we talk. 


      Following each correct answer is an explanation as to why the verb is a modal verb.

      1. must be > David is never late.
      2. can't think > I think it's a crazy idea, so I'm sure.
      3. must have arrived > I'm absolutely sure they arrived yesterday because I saw the ticket.
      4. could begin > It's possible, but I really don't know.
      5. can't have gone/couldn't have gone > David doesn't have money, so it isn't possible in my opinion.
      6. could live/might live, could be/might be > I know he likes big cities, but I don't know for sure.
      7. must have been, must have had > John's a fantastic singer, so I'm sure you had a great time.
      1. must get > I'm a teacher. I know!
      2. must be/might be > It's logical that she's looking for a job.
      3. must want > I know that she thinks about you often.