A reader asked for some guidance on using the modal verbs "may," "might," "can," "could," and "ought." Editor Emily Brewster responds:
The entries for each of these include the complete definitions, as well as many example sentences, so I will discuss here only the aspects of each that I think are likely to cause confusion.
These verbs are all modal verbs, which means that they are generally used in combination with other verbs, and are used to change the verb's meaning to something different from simple fact. Modals express possibility, ability, prediction, permission, and necessity.
"Ought" is probably the simplest of this set of modal verbs. It's almost always followed by "to" and the infinitive form of a verb. It means the same thing as "should," and is used in the same ways, although "ought" is less common and a bit more formal. A few examples of "ought" are "We ought to be home by noon," which means "I expect that we will be home by noon," and "I ought to fix that," which means "I should fix that."
"May," "might," and "could" can all be used to say that something is possible, as in "The story may/might/could be true" or "The painting may/might/could be very old." You can use any of the three in contexts like these.
"May" and "might" can both be used to say that one thing is true but that something else is also true, as in "This car may/might be more expensive than the other cars, but it will be cheaper to maintain." (If we used "could" instead of "may" or "might" here the sentence would mean that it is not certain that the car is more expensive than the other cars.)
Both "may" and "can" are used to indicate that something is allowed, but "may" is more formal: "You may leave whenever you like" is more formal than "You can go whenever you want to." Children are often taught that only "may" is used for permission, and that "can" is used only for ability. (For example, a child may ask a question like "Can I go outside?" and the responding adult might correct the child by saying that the child is able to go outside, but must ask permission by using "may.") "Can," however, is often used for permission.
"Can," "may," and "could" are all used to make requests. "May" is formal in these contexts, while "can" and "could" appear mainly in speech: "May I have your attention?" is more formal than "Can I have your attention?" or "Could I have your attention?"
Making matters even more confusing, I think, is that two of the modal verbs we're discussing are, aside from being modal verbs in their own rights, the past tense forms of two of the others: "might" can be used as the past tense of "may," and "could" is the past tense of "can." "Might" functions as the past tense of "may" mostly in formal contexts, as in "Generously, the senator inquired as to whether she might be of any help to us." "(The usual way of expressing past tense with "may" is "may" followed by "have" and a past participle. The sentence "It may take longer than they expect it to," becomes "It may have taken longer than they expected it to.") "Could" is the usual past tense of "can": "We could see the ocean from the window of the cabin." (see also the usage note at "can")
For further guidance on these verbs, check the entries for each.