The mistaken use of “affect” for “effect” and vice versa is one of the most common errors I come across in my reading. I have never had an issue with knowing which to use when, yet I have also never before tried to articulate for others a good guide toward making the correct choice between the the two words. It doesn’t help that each may be used as either a noun or a verb, although the more common usage of “affect” is as a verb and of “effect” is as a noun. According to Webster’s New World College Dictionary, 4th Ed., one of the definitions of “affect” (used as a verb) is “to have an effect on; influence, produce a change in.” It says that when used as a noun, “affect” refers to “an emotion or feeling attached to an idea, object, etc., or in general, emotion or emotional response.”
An “effect” (noun) is “that which is directly produced by an action, process or agent and is the exact correlative of ’cause.'” Used as a verb, “effect” means “to bring about; produce as a result … cause …” hmmmmmm. English can be a funny language.
Some sample sentences, with notes in parentheses as to whether affect or effect is operating as a noun or a verb:
“The movie “Star Wars” broke new ground in the arena of cinematic special effects.” (noun)
“The Governor hopes to effect change in the state’s policy on gambling, but legislative resistance may affect her ability to do so.” (verb, verb)
“The psychotherapist stated that the patient presented a flat affect when talking about the death of his brother.” (noun)