Affect, Effect - What's the Difference?
But when they're used wrong in writing, they brand the writer as not very knowledgeable about the language and perhaps -- though this may be unfair -- about his subject.
There's a simple rule that covers about 90 percent (a rough estimate that I wouldn't know how to verify) of their use, and I'll give you a memory device or two to help you with the rest.
For the vast majority of the time, "affect" is a verb and "effect" is a noun. If you "affect" something, it produces an "effect," just as "a" comes before "e" in the alphabet. "The recession affected the company's profits. The effect was a 14 percent decrease in sales."
When one of them appears as an adjective or adverb, it will almost always be "effect" -- "This toothpaste is effective in whitening teeth and preventing cavities." Or "The coach's defensive strategy effectively kept the other team from scoring." Both of these uses refer to an effect -- in the one case the effect on the teeth; in the other the effect on the opposing team's scoring.
So, for the 90 percent, "affect" is the verb and "effect" is the noun (with its team of adjective and adverb). For the other 10 percent (or whatever), they swap.
"Effect" as a verb means to bring a change of some kind to completion. You could say, "This toothpaste effects a whiter, brighter smile," meaning that it brings the whiter, brighter smile into existence. If the sentence said, "This toothpaste affects a whiter, brighter smile," it would mean that the whiter, brighter smile already existed, and the toothpaste just changed it somehow.
Here's another example of how the word choice can change the meaning. "The new administration effected a change in school policy." That's much deeper and more sweeping than, "The new administration affected a change in school policy."
In the first sentence, the administration made the change happen completely. In the second sentence, the change was already in place, and the administration just made it different somehow.
To show how the difference applies, let's say that the policy had to do with school uniforms. "Effect" could be a new requirement for school uniforms that goes into effect over whatever opposition might have come up. "Affect" would be that the new administration got to say whether the uniforms were blue or green or whether this school or that had to participate.
It might help to think of the verb "effect" as the end ("effect" starts with "e" like "end") of the verb "affect."
The noun "affect" -- short "a" and accent on the first syllable, so AF/fect -- is used mostly in psychology to describe a person's manner or appearance. A psychiatrist's evaluation of a new patient might say, "His affect was flat," which means the patient didn't show any emotion, or "His affect was excitable."
Branching out from there, this use of "affect" might appear outside of psychology to describe a person in a way that has a psychological feel to it, mostly literary fiction.
"Affect" and "effect" are hard for a lot of people, even some who consider themselves wordsmiths. The bottom line is that if you use "affect" as a verb and "effect" as a noun, the vast majority of the time you'll be right. The other uses are much more unusual, and you get extra credit for using them well.