Confusion between the words affect and effect is so common that I almost never see either of the words used correctly. Since I read anything that doesn't move fast enough to get away from me, and since I read hundreds of essays by college students each semester, I have reason to believe that this error is not just a misspelling, but an actual misapprehension of the two words and how they are used.
Generally speaking, affect is a verb and effect is a
noun. When you affect something, you produce an effect on it. Even in the passive voice, something would be
affected, not effected.
There are certain situations where effect is used as a verb and situations where affect is used as a noun, but very few people ever have a need to use them thus, so unless you are already confident of your ability to use these words correctly, just treat as general the rule that effect is a noun and affect a verb.
(If you feel the need to get fancy, however, here are the meanings of effect as a verb and affect as a noun. As a verb, effect means to execute, produce, or accomplish something; as a noun, affect is used primarily by psychologists to refer to feelings and desires as factors in thought or conduct.)
If you find that you have sometimes made the mistake of switching these two troublesome words, you probably should proofread specifically for them until you have formed the habit of using them correctly without having to think about it.
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