It’s easy to determine when to use the word can and when to use could. Can indicates ability. I can type a letter. I can run 10 miles. I can write a fan letter to Jon Hamm, although he probably won’t answer. Can denotes certainty. Could denotes uncertainty.
I could go to visit my sick neighbor if I don’t have to work on Thursday night. My neighbor could die from pneumonia if her immune system is not strong. My son’ s car could last another five years if he’s lucky. The most significant word in the last three sentences is “if” because the first part of every sentence depends on another factor.
It could happen, but maybe it won’t. Whereas when we use can, something will generally happen or at least the person has the ability to make it happen.
Parents used to teach children table manners by differentiating between the words can and may. A child would say, “Can I go now?” after dessert, and the parent would retort, “May I go.” Because clearly the child can go by simply getting up and leaving the table. Using may is a way of asking permission.
Sigrid Macdonald is the author of three books and two short stories, and is also a manuscript editor. Find her at http://sigridmacdonald.