First, notice how each word looks different from the other
affect starts with a
effect starts with e
Even though a should sound different from e, we pronounce these two words alike. Therefore, we often have difficulty deciding which one to use in our writing.
Affect
Affect means "to influence" or "to produce a change in."
Example of affect in a sentence
Since affect means "to influence" or "produce a change in" in this sentence, it is the correct word to use here.
Since the verb affect is related to the noun affection, sentences using affect often deal with changes in or influences on emotional states.
Example
Since affected (past tense) means "influenced" or "produced a change in" in this sentence, it is the correct word to use here.
Note: Affect is used as a noun in psychology to indicate one's emotional state or behavior.
Effect
While affect is always a verb, effect is usually a noun.
As a noun, effect means "the result," "the change," or "the influence."
As affect, a verb "produces a change," effect, a noun, is the "change" or "result."
Example of effect used as a noun
Since effect means an "influence" in this sentence, it is the correct word to use here.
Another example of effect used as a noun
Since effect means the "result" or "change" in this sentence, it is the correct word to use here.
Effect is sometimes, though not often, used as a verb, meaning "to cause."
Example of effect used as a verb
Since effect, a verb here, means "to cause," it is the correct word to use in this sentence.
Link to exercises on affect / effect.