Affect Is (Usually) a Verb
Before so much of the professional jargon of psychology found its way into the popular vocabulary, explaining the difference between affect and effect was a bit easier than it is now.
One could state categorically, “affect” is a verb:
The loss of his father affected him profoundly.
How will the new mall affect the neighborhood?
One would then explain that effect can be used as both noun and verb.
As a noun, effect means “the result of an action”:
What will be the effect of closing Main Street? (noun)
Have you read “The Effect of Gamma Rays on Man-in-the-Moon- Marigolds”? (noun)
As a verb, effect means “to bring about, cause, accomplish”:
The new administration effected many changes in policy. (verb)
The return to systematic phonics instruction effected the desired reading improvement within five years. (verb)
Now, thanks to the use of the word affect as a noun by psychologists and psychiatrists, we must consider this defintion:
affect (noun): Psychol. (and Psychiatry). A feeling or subjective experience accompanying a thought or action or occurring in response to a stimulus; an emotion, a mood. In later use also (usu. as a mass noun): the outward display of emotion or mood, as manifested by facial expression, posture, gestures, tone of voice, etc.
Examples of affect used as a noun:
The clinician observed the patient’s affect.
When the picture of a dog was flashed on the screen, Mr. Smith’s affect was sudden and violent.
Bottom line: It’s probably safe to say that in most everyday contexts, affect is used as a verb and effect is used as a noun. To decide which spelling you want, determine whether the word is being used as a noun or as a verb. If it is a noun (effect) it will probably have some kind of determiner or qualifier in front of it: the effect, an effect, some effect, any effect, the desired effect, etc.
Make sure to check our post Affect vs. Effect for words related to those terms.