"Affect" is almost always a verb meaning to influence or change something ("The rain affected our plans"). "Effect" is almost always a noun meaning a result ("The rain had no effect on our plans"). A quick trick: A is for Action (affect = verb), E is for End result (effect = noun).
These two words trip up even confident writers because they sound almost identical and both relate to cause and consequence — but they play different grammatical roles almost all of the time.
The basic rule
- Affect is a verb. It means to influence, change, or have an impact on something. "Lack of sleep affects your concentration."
- Effect is a noun. It means a result or outcome. "Lack of sleep has a serious effect on your concentration."
A trick to remember it
Think: Affect = Action (a verb, something being done), Effect = End result (a noun, the thing that happened). If you can put "the" or "an" in front of the word, you almost certainly want "effect" — "the effect", "a side effect".
The exceptions (why this trips people up)
Both words have rarer opposite uses that catch people out:
- Effect can be a verb meaning "to bring about" or "to cause to happen" — usually in formal writing. "The new manager effected several changes to the schedule." Here it means she caused the changes to happen, not that she was influenced by them.
- Affect can be a noun in psychology, meaning a person's outward display of emotion. "The patient showed a flat affect." This is specialist usage — you're unlikely to need it outside clinical writing.
For everyday writing, the simple rule (affect = verb, effect = noun) is right about 95% of the time.
More examples
- "The medication may affect your appetite." (verb — influences)
- "The medication's effects include drowsiness." (noun — the results)
- "How will this policy affect small businesses?" (verb)
- "What effect will this policy have on small businesses?" (noun)
