The primary use of ‘affect’ in the English language is as a verb. It means ‘to produce an effect.’ In other words, ‘to affect’ is synonymous with ‘to influence.’
The primary use of ‘effect’ is as a noun. It means a result or consequence. If you have a cause, it produces an ‘effect.’
Use ‘affect’ as a verb when you want to describe the way one item influences another.
INCORRECT: The loose rocks on the nature trail produced the affect of tripping the blond runner. (Here, ‘affect’ is incorrect. In this context, the noun ‘effect’ is the correct choice).
CORRECT: The loose rocks on the nature trail affect the blond runner’s stride by tripping up his feet. (Here, ‘affect’ functions correctly as a verb.)
Use ‘effect’ as a noun when you want to discuss the consequence or result of some action.
INCORRECT: When the blond man runs by in ridiculous short shorts, he effects those around him by making them laugh. (Here, ‘affects’ is the appropriate word because this context calls for the verb.)
CORRECT: The blond runner’s skimpy outfit produced a hilarious effect and his neighbors burst out in laughter at his appearance. (‘Effect’ is correct here because the context requires the noun.)
It is important to keep the difference between ‘effect’ and ‘affect’ straight because this is a very common error and it can make your writing look unprofessional. It is simple to correct if you take the time to memorize the difference in these words. To remind yourself that ‘affect’ is the verb, associate ‘affect’ with another word that starts with the same letter: ‘action.’ Also, remember that ‘cause and effect’ refers to a pair of nouns.