It's the age old dilemma. You're writing an email to a coworker and you have to stop to ask yourself, "Is it 'affect' or 'effect'?"
These words sound the same, and have similar meanings. Both can function as a verb or a noun, which means you can’t use parts of speech to distinguish them. They’re like identical twins — they might seem almost alike at first glance, but once you get to know them, they’re not the same. So what makes them different?
Affect/Effect: The Noun Form
Let’s start with the noun versions of "affect" and "effect." By definition, "effect" as a noun means something that’s created. If your friend is surprised when you jump out at them wearing a scary Halloween mask, that action has the effect of scaring them. "Effect" as a noun can also mean something exists. If you say “that law is still in effect,” you’re saying that law still needs to be obeyed.
"Affect" as a noun is more abstract, and it's mostly limited to the psychology field. As a noun, "affect" refers to a show of emotional expression. You could say someone has an excited affect (or none at all) when they’re surprised. The thing is, unless they're a mental health professional (or perhaps a novelist or essayist), most people choose a different word for this context, so you’re not likely to encounter "affect" as a noun in everyday conversation.
Affect/Effect: The Verb Form
The two words get a little closer as verbs, but the actions they perform have different effects (see what we did there?).
"To affect" has a tangible meaning — it refers to a material change or influence on an object or concept. You're not affected by an earthquake a thousand miles away, but you would be affected by your coworker quitting and leaving all their work to you.
"To effect," on the other hand, is kind of like the end game of that measurable influence. It refers to an accomplished goal, or the cause for existence. You can say you want to effect change in the world — you want to create change as an end result. But "effect" as a verb is less common, because most of the time people use “create” or a similar verb.
It might seem confusing, but once you use "affect" and "effect" in their different forms, you’ll be able to tell them apart easily. It might be helpful to remember most of the time, "affect" is used as a verb and "effect" is generally a noun. There are exceptions, but this guideline is generally true for conversational speech.