Affect vs. Effect

This pair trips up more writers than almost any other, and the fix is one sentence: affect is usually a verb, effect is usually a noun. Something affects you; what you are left with afterwards is an effect.

Affect: the action

To affect something is to act on it, change it or influence it. If the word in your sentence is doing something, you almost certainly want affect: the weather affects my mood, the strike affected deliveries, will this affect my grade?

Effect: the result

An effect is a thing – the outcome the action leaves behind. Because it is a noun, it usually follows words like the, an, no, this or side: the effect was immediate, a side effect, no effect at all. If you can put the in front of it, use effect.

The two rare exceptions

English wouldn't be English without exceptions, but these are genuinely rare. Effect can be a verb meaning to bring something about, almost always in the phrase to effect change. And affect can be a noun in psychology, where it means a person's displayed emotion. Unless you are writing policy memos or clinical notes, you can ignore both and lean on the main rule.

The quick test

Swap in another word. If influence (the verb) fits, write affect. If result fits, write effect. “The rain influenced our plans” works, so it's affected. “The result of the rain” works, so it's the effect.