Affect and effect come from the same Latin root. They are practically unrelated in meaning, yet differ only by their first letter. Since the accent is on the second syllable, they sound nearly identical in speech. Usually, affect is used as a verb and effect as a noun. (The spelling affects the meaning, but the effect of the similarity is confusion.) That would be bad enough, but affect can be used as a noun and effect can be used as a verb. Not only that, but adding suffixes to both words creates new pairs differing in spelling by only one letter yet having very different meanings. Looking them up in a dictionary is easy enough, but it helps to look at each pair separately.
Affect (noun): a term used in psychology referring to emotion or feeling as opposed to cognition or action. As a technical term, people outside the field of psychology are not likely to use it, but many non-psychologists read about psychology.
Effect (noun): something that ensues from a cause, a result; how something acts upon or influences something else; the final outcome; the ability to achieve a desired result; the condition of being in full force, as the date on which a law goes into effect; the impression made on a hearer, spectator, etc. by manner of presentation; the basic meaning of something written or said, as in "something to that effect" or "in effect"–and there are more.
Effects (plural noun): physical belongings; the sounds, lighting, or other artistic techniques accompanying a movie, broadcast, or play.
Affect (verb): to influence or bring about a change, as in how weather affects driving or how it affects someone’s mood. Here is one case where the two spellings are related: Whatever affects something else (verb) produces an effect (noun); to feign or pretend to feel or have; to make a show of a liking or disliking.
Effect (verb): to cause; to produce as a result; to bring about; to accomplish.
Several other words are derived from affect: affectation (noun), affected (adjective with two meanings), affecting (adjective), affection (noun) and affectionate (adjective). There are no corresponding words derived from effect. Likewise, nothing comparable to effectual (adjective) or effectuate (verb) is derived from affect. There is one remaining troublesome pair.
Affective (adjective)–again, used only in psychology: pertaining to, resulting from, or arousing emotions or feeling.
Effective (adjective): serving a purpose; having the desired result; ready to be used; operative, or in effect.
By the same author: Going on a "which" hunt: choosing between "that" and "which" in relative clauses
