Modals (perhaps you learned them in school as a category of auxiliary or "helping" verbs) are among the most confusing English words to master. And no wonder: besides their refusal to follow the patterns verbs normally take, we also use some of them interchangeably so regularly that we forget they are not identical in meaning. Can and may are two of the English modals that have fallen into that pattern.

The basic difference between can and may is pretty simple. Can is about ability; may is about permission*. Consider these two examples: John is a piano prodigy who can flawlessly recreate a full concerto after hearing it only once. John may play the piano after he finishes the liver-and-onion casserole his mother made for dinner.

In the first sentence, the focus is on John's tremendous capability: he CAN. In the second, John's ability to play is beside the point; under what condition he is allowed to exercise his talent is the question: he MAY.

When you're not sure which to use, jog your memory by thinking of these two famous instances.

Sammy Davis, Jr.'s song "Yes I Can" = ABILITY
vs.
the children's game "Mother, May I?" = PERMISSION

Try distinguishing the difference with a few example sentences. Complete each one with either can or may.

1) A talented chef ___ prepare a delicious cake without using any dairy, egg, or wheat products.

2) ___ I borrow your car while you are on vacation?

3) Passengers ___ not walk around the cabin while the "Fasten Seatbelts" sign is on.

4) If he takes a running start, Phil ___ perform a long jump of over twelve feet.

5) The boss knows Wilma ___ do the job; it requires the skills Wilma demonstrated in her old position.

Answers: Sentences 1, 4, and 5 require can; may completes 2 and 3.

English speakers are often guilty of using can in both instances. In everyday speech, chances are most people wouldn't even notice the substitution. The Chicago Manual of Style (15th Edition) acknowledges that, in informal situations, can is often used to indicate permission in the same way may is above. However, Chicago also offers a caveat: in any kind of formal writing, may to express permission is far preferable to can. Likewise, in a speaking situation other than casual conversation, it would be prudent to consciously use the correct modal.

*May has a second usage as well: an expression of possibility. (Ex: If conditions are right, I may go fishing this weekend.) However, that usage is more commonly confused with might than with can--a problem which will be addressed separately in a future article.

AnnaLisa Michalski writes and publishes the ezine Word-wise. She owns and operates Admin Maven, a virtual assisting service specializing in proofreading and copy editing. http://www.adminmaven.com/