Reading someone else's prose as closely as an editor must can be an intimate experience. An editor gets to know the characteristic rhythms of the writer's prose, the favorite turns of phrase, the verbal tics. The verbal tics can tick an editor off, though, and the writer's favorite turns of phrase are not always the editor's.
Going through a manuscript the other day, I noticed that I had replaced the words "has the ability to" with "can" about a dozen times in a chapter, and it began to grate. "Has this man lost his acquaintance with this very useful simple word?" I wondered.
He's not alone. A quick Google News check turns up plenty of instances of this "ability" construction where "can" would do. But there are some jobs in the business of building sentences that can just can't do.
For all its hard work and can-do attitude, can is what grammarians call a "defective verb." It's missing some pieces. It can't be taken through a full conjugation, like sing, dance, or for that matter, prognosticate. It doesn't have a future tense, for instance.
We say, "I can ride a bicycle," in the present tense. Or in the past: "When his eyes got used to the dark, he could see the approach of the enemy soldiers."
The Macmillan dictionary offers this nuance: Could as the past tense of can signals ability to do something. But to refer to someone actually doing it, the dictionary advises the "was/were able to" construction.
Could can be a conditional: "If he let us have his car, we could go to the beach."
But there's no future of can. You have to use some form of "to be able" and an infinitive. "If they save enough money, they will be able to buy a house." You don't have to say, though, "they will have the ability to buy a house."
Can also lacks an imperative. You can tell someone "Stop!" or "Go!" but you can't tell someone "Can!" Can lacks a gerund, too – that "ing" form that makes a verb a noun. There is such a thing as canning, but that happens in the kitchen.
If can has no future, must is in even worse shape. It has neither a past nor a future form.
The present-tense urgency of must yields to much less energetic "have to" constructions in the past and future: "I must get an answer now" has a lot more oomph than "I had to get an answer."
Can and must are among the most common defective verbs in English, along with other so-called modal verbs, such as ought, may/might, and the will/would and shall/should duos. They are used as "auxiliary" or "helping" verbs to convey nuances of ability and possibility.
Grammar nerds point out that beware is another defective verb, used almost exclusively in the imperative. Quoth, which lives on in the language only in its past tense to mean "said," is often used mock archaically.
Able and ability come from roots in Latin.
But can is an Old English word related to the idea of "knowing"; "I can ride a bicycle" is thus "I know how to ride a bicycle." It's related to canny, as in the proverbial "Canny Scot" and cunning.
Cunning was an adjective meaning "learned or skillful" in the early 14th century before it came to mean skillful deceit, according to the Online Etymology Dictionary.
With all these associations, can has been conveying ideas of ability and knowledge for centuries. But there are some jobs that can, as a defective verb, just can't do.