The basic rule
Affect is (in the vast majority of cases) a verb meaning "to influence" or "to have an impact on": "The weather affected our plans." Effect is (in the vast majority of cases) a noun meaning "a result" or "an outcome": "The medicine had a strong effect."
The easy trick: RAVEN
Remember RAVEN: Remember Affect = Verb, Effect = Noun. If you need a verb, reach for "affect." If you need a noun (often after "the," "an," or "this"), reach for "effect."
The exceptions (less common, but real)
"Effect" can also be a verb meaning "to bring about" or "to cause to happen": "The new CEO effected major changes." "Affect" can also be a noun in psychology, meaning an observed emotional expression: "the patient showed flat affect." These uses are rarer — for everyday writing, the verb/noun rule above covers 95% of cases.
Examples
Verb (affect): "Stress can affect your sleep." "The delay affected everyone's schedule."
Noun (effect): "The effect of the policy was immediate." "Side effects of the medication."
Frequently asked questions
What's the fastest way to remember affect vs. effect?
Use RAVEN: Affect = Verb, Effect = Noun. That covers most everyday sentences.
Can "effect" ever be a verb?
Yes — "to effect" means "to bring about," as in "effect change." It's formal and less common than the noun use.
Can "affect" ever be a noun?
Rarely, mostly in psychology, where it refers to an observable emotional expression.
Is "side effects" spelled with affect or effect?
"Side effects" — it's a noun (a result of taking medication), so it uses "effect."