Most commonly, when you’re using a noun, it will be effect. When you want a verb, it’s usually affect. But to be fair, I have to admit that both can be used two ways. Let’s start with the most common usage for both.
Affect: The Verb
To affect something is to act on it and change it somehow:
The weather affected her mood.It can also mean to touch or move someone emotionally—as in, to rouse affection:
(It changed her mood, or at least influenced it in some way. It was sunny and that made her happy; or maybe it was rainy and that made her angry.)
His letter affected me deeply.Effect: The Noun
An effect is a change brought about by something else.
I hope Dad’s lecture had the right effect on you.It can also mean an impact or impression, as in special effects or
Within minutes Angie felt the effects of the poisoned dart.
The actor’s soliloquy had an immediate effect on the audience.
The whole street blazed with Christmas lights; the effect was spectacular.Here are the less common ways of using both words:
Wanda paused for dramatic effect before announcing the murderer.
Affect: The Noun
This is the least common use of the four meanings we’ll discuss. A person’s affect is the outwardly demonstrated manifestation of his or her emotional state. Psychiatrists use this term a lot; how is a patient acting? What is her demeanor? Does she seem calm, happy, distressed, angry? What can we see on her face that reflects her emotions? That’s her affect.
Effect: The Verb
In this sense, effect means to bring about, to cause to come into being, or to accomplish:
Throughout his long career, the senator effected real change.