Modal verb problems

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This page summarizes common problems that second language learners have with English modal verbs, particularly East Asian learners. For a more about modal verbs themselves, including a comprehensive guide to meanings and usage, please see the modals page.

1 Should & must

In the history of the English language, should developed as a conditional or hypothetical form of shall (future tense). Its usage differs from that of must; compare the following.

  1. For this reason, online banking must be quick and simple.
  2. For this reason, online banking should be quick and simple.
  3. He should clean the lab.
  4. He must clean the lab.
  5. He should have cleaned the lab.
  6. He must have cleaned the lab.
  7. He should be a conscientious labmate.
  8. He must be a conscientious lab mate.

Must is used to express a strong obligation (like a moral or social obligation, or otherwise some kind of obligation that is an external force). We can thus say that it is (1) external to the speaker or the sentence subject, and it is (2) some type of obligation, such as a moral or social obligation.

The verb should expresses strong advisability, a strong expectation, or a warning. So in the following sentence, ‘must’ would sound odd; ‘should’ or ‘ought to’ would be better here.

  • For this reason, online banking must be quick and simple. (Odd, unless this is the bank manager essentially commanding the IT department to improve the system.)
  • For this reason, online banking should be quick and simple. (What a bank customer might say.)

Here, it would be better to say that it “should be quick and simple,” as a strong request or advice to someone; it would be hard to view this as some type of social or moral obligation that banks should fulfill (unless, e.g., a bank manager is criticizing his/her own bank or giving an implied command to someone to fix it). The semi-modal ought to is very similar to should. The verb should has a more external nuance, e.g., it is not just my feeling, but probably what we all think, while ought to is more internal or subjective. The nuances are rather slight, and often these would be interchangeable with little or no real difference, except that ought to can sound more informal, since it is more subjective.

  • I should go see them. (I sense that this would be advisable or expected of me.)
  • I ought to go see them. (This is just my own feeling.)

Finally, must and must have have an additional meaning of inferred probability, where the speaker assumes that X must be the case--this is the necessary explanation for what s/he sees.

  • You must be crazy. (= That has to be the explanation for your behavior.)
  • He must have lied on his job application. (= This has to be the explanation for his incompetence, though he claimed to have the right experience for the job.)

2 Could

The modal could developed originally as a past tense form of can. In contemporary English, it still has this meaning, but its primary or default meaning is different. Compare the following.

  1. When I was young, I could lift 40 kg fairly easily.
  2. We could get one meter of snow today.
  3. She could be gone for lunch, but I’m not sure.
  4. She couldn’t have stolen it – she’s not like that.
  5. Could I borrow your car tomorrow?
  6. We could perform the experiment--if we had access to the right population of participants.
  7. If we could determine the applicants’ first language based on previously provided information, then this would allow for inclusion of a helpful predictor in the model.
  8. We could replicate the experiment by using a slightly different substrate material.

The verb can puts more emphasis on something possible, though perhaps requiring effort or involving difficulty in achieving it. The verb could is more hypothetical in meaning. The default meaning of could is hypothetical, potential, or conditional states or actions, rather than as a past tense of can. It historically developed in English as a past tense of can, but its main use in modern English is for hypothetical situations or conditional (if-type clauses) clauses. For example, trying to use could as a past tense form of can / be able to in this sentence sounds awkward or ambiguous.

  • We could replicate the experiment with subjects from a different country.

This is ambiguous, because native English readers would first interpret this as a hypothetical statement – “hypothetically, we might try to replicated it” or “hypothetically, we might be able to replicate it.” Rather than using could here, it would be better to either use was able to to emphasize the idea of ability, or to delete it altogether and use just the main verb, and/or the main verb modified by an appropriate adverb.

  • We were able to replicate the experiment with subjects from a different country.
  • We replicated the experiment with subjects from a different country.
  • We successfully replicated the experiment with subjects from a different country.

Could is also used as a more formal form of can for asking or giving permission.

  • Can I go now?
  • Could I borrow your pencil?

However, in some cases could can be unambiguously used as a past form of can: (1) if the context is clearly past tense, for example, with a past time expression or a clause in the past, and (2) in a negative expression. In such cases, it is less ambiguous, so could not would be more common and clearer. Otherwise, could is more commonly used in conditional statements, for hypothetical and speculative statements, or for permission.

  • When I was young, I could lift 40 kg fairly easily.
  • I tried, but I couldn't lift it.

3 May vs. might

The verb may expresses as its core meaning the idea of possibility.

  • He might be lying to us.
  • This might be an interesting film.

An extended meaning is for expressing permission, but this is rather formal.

  • May I leave now? (very formal)
  • Could I leave now? (formal)
  • Can I leave now? (informal)
  • You may leave / may not leave now. (more formal)
  • You can / cannot leave now. (more informal)

The verb might indicates possibility, but with a sense that is more remote or less certain than may.

  • He might agree to it (but then, he might not).

It can also express speculation, suggestion, or implied criticism; in very formal British English, it can be used for requests.

  • She might have bought it in Mongolia, but I’m not sure if she’s been there. (speculation)
  • I’m not sure what the problem is, but you might try putting the RAM chip in a different slot. (suggestion)
  • You might have asked if someone else wanted cake before eating the last piece! (criticism)
  • Might I have another glass? (British style, for permission)

4 May, might & could

Consider the following examples.

  1. The President appeared on TV and angrily denied the accusations that he had had an affair with a female intern. He emphatically stated, “I did not have sex with that woman.” Members of the public thought that he _________ be lying. Psychologists who study body language and facial expressions examined the video of his talk to analyze his expressions. They concluded that he _________ be lying.
  2. The President mislead us by claiming that Iraq had weapons of mass destruction, as an excuse for going to war. In the end, there were no WMDs, and the war was a tremendous mistake that cost hundreds of billions of dollars, thousands of lives, and the ongoing stability of Iraq. When we remember the President giving his speeches to Congress and to the nation, we look back and conclude that he _________ lying.

As noted above, may sounds slightly more probable than the more remote might. These two verbs differ also from the hypothetical sense of could in terms of one nuance. The nuance of may, might is external, that is, the speaker is focusing on what is external to the subject, such as the situation or circumstances. Withe the presidential example, the speaker draws a conclusion by focusing on the suspicious situation, circumstances, or events. The nuance of could is internal to the subject, that is, the speaker makes this conclusion about whom s/e is speaking--the sentence subject. This comes from the fact that could derives from can, a verb of ability. Here, perhaps the speaker does not trust the president, and believes that it is in his ability or nature to lie, or that he has lied in the past. The speaker assesses not so much the situation, but the subject--the president himself.

The forms may have, might have, could have are similarly used for speculating about the past.

  • She may have escaped (but we’re not sure).
  • She might have escaped (but not so likely).
  • She could have escaped (it’s possible, given her nature or abilities).