Affect is usually a verb meaning to influence or have an impact on something. Effect is usually a noun meaning the result or outcome of a change. For example: “The weather affected her mood” (verb); “The effect of the weather was a low mood” (noun).
The confusion arises because both words relate to the same concept—cause and change—but play different grammatical roles. There are rare exceptions: “effect” can be used as a verb meaning to bring about (“to effect change”), and “affect” is used as a noun in psychology to describe emotional expression. When in doubt, test the sentence by replacing the word with “influence” (verb) or “result” (noun) to see which fits. For more grammar clarity tips, read our guide on passive voice vs. active voice. Quetext’s Grammar Checker flags affect/effect misuse automatically.