Affect is most often a verb meaning to influence or change something. Effect is most often a noun meaning the result or outcome of a change. The common test is: if you can replace the word with influence, use affect; if you can replace it with result, use effect.

Use affect when one thing influences another, often without being the direct cause of a final outcome. It answers the question What does it change or influence. It commonly appears in academic, medical, business, and everyday contexts when describing impact on mood, behavior, performance, or conditions.

Rule	Example
Use affect for influence on a person or thing	Noise can affect concentration.
Use affect with measurable impacts	The storm may affect flights.
Use affect for changes in feelings or mood	The news affected him deeply.

Use effect for the outcome or end result of an action, event, or cause. It answers the question What happened because of it. It commonly appears with verbs like have, produce, see, feel, and with adjectives describing results.

Rule	Example
Use effect for the result of a cause	The effect of the storm was widespread.
Use have an effect on for impact phrased as a noun	Stress can have an effect on sleep.
Use side effect for secondary results	Drowsiness is a common side effect.

A practical way to choose is to try a synonym in your sentence. If influence fits, choose affect. If result or outcome fits, choose effect. This method works for most everyday writing and speaking.

Rule	Example
If influence fits, use affect	Heat can affect batteries.
If result fits, use effect	The effect was immediate.

Certain word partnerships strongly prefer one form. Learning these chunks helps you choose quickly and sound natural. Many of them are fixed phrases used in formal and informal English.

Word/Phrase	Definition	Example
affect the outcome	influence the final result	Small errors can affect the outcome.
have an effect on	produce an impact on	Caffeine has an effect on alertness.
take effect	begin to work or apply	The new rule takes effect tomorrow.
in effect	currently operating or true	The policy is still in effect.
to that effect	expressing something similar	She said something to that effect.

Effect can be a verb meaning to cause something to happen or to bring something about. It is more formal and often used with nouns like change, reform, solution, and improvement. Use it when someone successfully makes a planned result happen, not just influences it.

Rule	Example
Use effect meaning bring about	The new CEO effected major changes.
Often pairs with nouns like change or reform	The policy aims to effect reform.

Affect can be a noun in psychology meaning outward emotional expression, such as facial expression, tone, or visible mood. This meaning is specialized and mostly appears in clinical or academic settings. In everyday English, people usually say mood or emotion instead.

Rule	Example
Use noun affect for observable emotional display	The clinician noted a flat affect.
Prefer mood or emotion in general contexts	His mood seemed low today.

Affect emphasizes the process of influence or change, while effect emphasizes the final outcome. If you want to highlight what caused a shift, affect is usually the right choice. If you want to highlight what happened as a consequence, effect is usually the right choice.

In everyday writing, you will mostly use affect as a verb and effect as a noun. The verb effect and the noun affect are correct but less common, so they can sound formal or technical. Choosing the common forms improves clarity unless you specifically need the specialized meaning.

Rule	Example
Default to affect verb and effect noun for clarity	Poor lighting affects photos and the effect is noticeable.
Use effect verb in formal contexts	The committee hopes to effect change.
Use affect noun in clinical contexts	Restricted affect may indicate depression.