Few words separated by a single letter cause as much confusion as “affect” and “effect.” Beyond their superficial similarity, the water is further muddied by the fact that each functions as a verb and noun. None of these forms is interchangeable, so keeping them straight is important. Let’s begin with some broad definitions:

Affect (ǝ-FĚKT), as a verb, has two discrete meanings: to influence or to simulate, as in “The storm affected turnout at Richardson Auditorium” or “Professor James affected the dress and manners of an Ewardian don, much to the amusement of his students.”

Affect (ĂF-ĕkt), as a noun, is a psychological term referring to the emotions, as in “The staff of McCosh Health Center were alarmed by her gloomy affect.” Note the stress on the first syllable.

Effect (ĭ-FĚKT), as a verb, means to bring about, as in “The dean effected significant improvements in student behavior during her tenure.”

Effect (ĭ-FĚKT), as a noun, signifies a result, as in “What effect did the recession have on Nassau Street’s small businesses?”

If these distinctions are only moderately helpful, bear this in mind: in most, but not all, cases, if your sentence calls for a verb, use “affect,” and if it calls for a noun, use “effect.”

Uncertainty is most likely to arise when a sentence could reasonably accommodate the verbal forms of either “affect” or “effect,” and here you need to remind yourself of their distinct meanings. Do you wish to say that one thing has influenced another (affect) or that one thing has brought another about (effect)? The American Heritage Guide to Contemporary Usage and Style offers this example: “Using effect in the sentence The measures have been designed to effect savings implies that the measures will cause new savings to come about. Using affect in the very similar sentence These measures will affect savings implies that the measures will cause a change in savings that have already been realized.”

With practice, it won’t be long before you’re churning out sentences like this: “My psychiatrist told me that the effects of the short winter days had affected me, effecting changes in my affect, so to please her, I affected a sunny persona.” Isn’t English great!