To decide between "effect" and "affect," determine if the word is being used as a noun or a verb. "Effect" is typically used as a noun, while "affect" is typically used as a verb. Another method is checking if the word's definition can logically be substituted into the sentence.
When "effect" is used as a noun, it means "outcome" or "consequence" and is often preceded by an article or an adjective. For example, a sentence may use the words "the effect" or "positive effect." When "effect" is used as a verb, it means "to bring about."
When "affect" is used as a verb, it means "to produce a change." Although it's almost always used as a verb, it is also used as a noun to describe an emotional state or a facial expression.
To substitute the word's definition into the sentence, replace "effect" or "affect" with the word's definition. Use the definition for the correct part of speech. For example, if you're using "effect" as a verb, use "brings about" instead of "effect." If the sentence is still logical, you're using the correct word.
"Effect" and "affect" are homonyms, which are words that sound similar or exactly the same but have different meanings.