Affect vs. effect: which one do I want?

The rule that covers about 95 percent of real usage: affect is a verb meaning to influence, and effect is a noun meaning a result. The weather affected my mood; the weather had an effect on my mood. The mnemonic RAVEN, Remember, Affect Verb, Effect Noun, has carried generations of writers, and swapping in the word influence is a good test: if influence fits as a verb, you want affect.

The exceptions exist and are rarer than people fear. Effect works as a verb meaning to bring about, as in effect change, a favorite of press releases. Affect works as a noun only in psychology, where flat affect describes diminished emotional expression.

If memorizing exceptions is not your hobby, here is the practical cheat: when you are describing an action, write affect; when you can put the or an in front of it, write effect. That heuristic alone will keep you right in nearly every email you ever send.