I'm teaching English in Japan. I'm using "Grammar In Use Intermediate Workbook" (Cambridge). The following is the question in the workbook:

Complete the sentence. Use can, could, must or (be) able to + verb in parentheses.

  • A: You have chest pain? I'll take you to the emergency room. It ( ) (be) something serious.

The workbook says the correct answer is "could".

But students said to me that "must" and "can" also seem to be correct.

I also think that the correct answer will change depending on the context. Could you kindly give me some advice if we can use both "must" and "can" in this situation? What's the difference, and how can I explain it to the students?

  • 2
    As a native speaker, "could" sounds right to me. Using "must" could fit, though it would imply it is almost certainly something serious, which isn't indicated by the example sentence provided. Using "can" doesn't fit the sentence here, though saying "that can be something serious" instead of "it can be something serious" works. It just doesn't work with the structure provided. Jun 2, 2015 at 7:35
  • Agree with Alexis: we would never say the line with "can". I read "must" not as a medical judgment, but possibly unjustified subjective expectation on the part of the speaker. Did no one mention "might"? That would be very possible to a native, totally synonymous with "could"?
    – David Pugh
    Jun 2, 2015 at 8:27
  • There is too little evidence to say with absolute certainty that the pains in the man's chest "must" be serious. Not all chest pains are related to heart failure, sometimes it could just be heartburn, severe indigestion, a panic attack etc. The modals could or might suggest that the speaker is unsure if the incident is serious, but to be on the safe side, it's best to visit the A&E (BrEng) ER (AmEng)
    – Mari-Lou A
    Jun 2, 2015 at 17:08
  • The only one you definitely cannot use in your context as given is: be able to
    – Lambie
    Dec 4, 2023 at 18:47

1 Answer 1


1) Use "must "to show that something is very likely.
2) Use "could" to express possibility.

I think "could" is the best option, and you should avoid using "can" here.

You have chest pain? I'll take you to the emergency room. It could be something serious.

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