The majority of the time you use affect with an a as a verb and effect with an e as a noun.
When Should You Use Affect?
Affect with an a means “to influence,” as in, “The D in Math affected Bill’s GPA,” or “The rain affected Amy’s hairdo.” Affect can also mean, roughly, “to act in a way that you don’t feel,” as in, “She affected an air of superiority.”
When Should You Use Effect?
Effect with an e has a lot of subtle meanings as a noun, however the meaning “a result” seems to be at the core of all the definitions. For example, you can say, “The effect was eye-popping,” or “The sound effects were amazing,” or “The rain had no effect on Amy’s hairdo.”
Rare Uses of Affect and Effect
Of course, these rules are not always followed. Affect can be used as a noun when you’re talking about psychology–it means the mood that someone appears to have. For example, “She displayed a happy affect.” Psychologists find it useful because they know that you can never really understand what someone else is feeling. You can only know how they appear to be feeling. And, effect can be used as a verb that essentially means “to bring about,” or “to accomplish.” For example, you could say, “The instructor hoped to have a positive effect within the classroom.”