Like many things in life, using affect and effect correctly seems simple at first. Then complications develop. However, if you follow one basic guideline, you’ll be right most of the time. Affect usually means “to influence.” Effect usually means “result.”
Affect is most often used as a verb; that is, to show action. If something changes, people might ask, How will that affect me?
Effect is often used as a noun; that is, to name something. If something changes, people might ask, Will that have a good or bad effect?
That’s fairly straightforward. To test whether you’re using affect correctly, replace with influence. For example, the question Can lack of sleep affect performance? becomes Can lack of sleep influence performance? The result makes sense, so affect is correct.
The problem is that both words can be used as either verbs or nouns. Although affect is rarely used as a noun in everyday conversations, clinicians use the term to describe a person’s level of emotional response: The diagnosis is based on the patient’s flat affect. If you replace affect with influence here, the result is nonsense: The diagnosis is based on the patient’s flat influence.
When effect is used as a verb, it means “to cause”: The drug effected a complete cure. To test whether effect is correct, replace it with cause: The drug caused a complete cure. The result makes sense, so this usage is correct.
Want more examples? You’ll find several, along with clear explanations, at Professor Malcolm Gibson’s Wonderful World of Editing site: http://web.ku.edu/~edit/affect.html
