effect and affect - the difference
 
Effect is a noun meaning outcome, consequence, or appearance.

To affect is a verb meaning to transform or to change.
 

Difference between Affect and Effect

There is often confusion over the words effect and affect. In order to understand which to use, you must know the difference between a noun and a verb.

Effect is a noun. Affect is a verb. If you're not confident with spotting nouns and verbs, there are workarounds to help to choose the right one. (See Hot Tip on the right.)

Examples:

What effect did foot-and-mouth disease have on your business?
(effect – noun)
(Tip: Try substituting the noun effect with the noun consequence to confirm it's a noun.
"What consequence did foot-and-mouth disease have on your business?" < sounds ok; therefore, effect is correct)

Did foot-and-mouth disease affect your business?
(affect – verb)
(Tip: Try substituting the verb affect with the verb transform to confirm it's a verb.
("Did foot-and-mouth disease transform your business?" < sounds ok; therefore, affect is correct)

Do not allow this incident to effect your decision.  
(Tip: Do the substitution test.
"Do not allow this incident to consequence your decision" < nonsense; effect
is wrong.
"Do not allow this incident to transform your decision" < sounds ok; affect is correct.)

That spiral effect is effecting my eyes.
(Note: Sometimes, the noun-substitution test won't work with consequence because effect is quite a versatile word. You might have to try other nouns, e.g., appearance. If you find yourself trying to use this word as a verb (e.g., appear, appears), then you should be using affect not effect.
 
Select the correct version:



 
 



 
A LITTLE TRICK TO SPOT EFFECT 

The word effect has several meanings. It can mean outcome, consequence, or appearance. Try using one of these instead of effect. If the sentence still makes sense, then effect is almost certainly correct.
(This trick works because effect is a noun, just like the words outcome, consequence, and appearance.)

A LITTLE TRICK TO SPOT AFFECT 

Try using the verb to transform(in its various forms, e.g., transforming, transformed, transforms) instead of affect. If the sentence still makes sense, then affect is almost certainly correct. However, if you find yourself trying to use transformation, then you should be using effect because both are nouns.
(This trick works because to transform is a verb, just like to affect. )

NO CONFUSION

There should be no confusion with affecting or affected. These are always verbs.

BE AWARE

There is a verb to effect. It is quite rare, but useful in business writing. It means to bring into being.

The new policy will be effected as soon as the paper is signed.
 

See also:

What are nouns?
What are verbs?
List of easily confused words