Grammar Girl here.
Today's topic is when to use affect with an a and when to use effect with an e.
This question is from Eric on the voicemail line.
"I have a question for you regarding the "affect or effect" conundrum. Please shed some light on this for me. I've read style guides but I can never remember, so I'm looking for some kind of mnemonic or something that might help."
This has been by far the most requested grammar topic. In addition to Eric, at least seven people have asked by e-mail, so I have a mnemonic and a matching cartoon to help you remember.
Before we get to the mnemonic though, I want to explain the difference between the two words.
It's actually pretty straightforward. The majority of the time you use affect with an a as a verb and effect with an e as a noun.
Affect with an a means "to influence," as in, "The arrows affected the aardvark," 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."
Effect with an e has a lot of subtle meanings as a noun, but to me 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," or "The trick-or-treaters hid behind the bushes for effect."
So most of the time affect with an a is a verb and effect with an e is a noun. There are rare instances where the roles are switched, but this is "Quick and Dirty" grammar, not comprehensive grammar, and I don't want to confuse you. My impression from your questions is that most people have trouble remembering just the basic rules of when to use these words, so we're going to stick with those, and you'll be right 95% of the time.
So, for our purposes, affect with an a is a verb and effect with an e is a noun; and now we can get to the mnemonics. First, the mnemonic involves a very easy noun to help you remember: aardvark. Yes, if you can remember aardvark -- a very easy noun -- you'll always remember that affect with an a is a verb and effect with an e is a noun. Why? Because the first letters of "a very easy noun" are the same first letters as "affect verb effect noun!" That's a very easy noun. Affect (with an a) verb effect (with an e) noun.
"But why aardvark?" you ask. Because there's an example to help you remember as well. It's, "The arrows affected the aardvark. The effect was eye-popping." It should be easy to remember that affect with an a goes with the a words arrow and aardvark, and that effect with an e goes with the e word eye-popping. If you can visualize the sentences, "The arrows affected the aardvark. The effect was eye-popping," it's pretty easy to see that affect with an a is a verb and effect with an e is a noun.

And you absolutely must go to the transcript at quickanddirtytips.com this time, even if you've never been there before, because I commissioned a cartoon of the example, and it turned out great. It's an aardvark being affected by arrows, and I think that looking at it will help you remember the example sentences; and it's cute. So if you go to quickanddirtytips.com you can print it out and hang it by your desk, or even order a T-shirt or a mousepad if you really want to. (Also, I'll write about the rare opposite uses of affect and effect at the end of the transcript for those of you who are curious.)
So a very easy noun will help you remember that affect with an a is a verb and effect with an e is a noun, and the example will help you see how to use both words in a sentence.
That's all. As always this is Grammar Girl, striving to be your friendly guide in the writing world. Here's a big thank you to the listeners who've taken the audience survey; it's still available for those of you who haven't done it yet. And also, a big thank you to the listeners who made donations using the PayPal button on the website. It was your donations that allowed me to commission the cartoon, and I really appreciate it.
Finally, Andy called me on the voicemail line to let me know that the Schoolhouse Rock videos are available at iTunes. So you can still buy the whole DVD at QuickAndDirtyTips.com, but you can also buy just your favorite episodes at iTunes, which I'm sure some of you will want to do!
Rare Uses of Affect and Effect
Affect can be used as a noun when you are 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 know what someone else is feeling. You can only know how they appear to be feeling. You can find more on psychological affect at Wikipedia. (I'm not an expert in psychology, so please correct me if happy isn't a valid affect.)
Effect can be used as a verb that essentially means "to bring about," or "to accomplish." For example, you could say, "The aardvark hoped to effect change within the burrow." You can find more on effect at Dictionary.com.
Thanks to Len from the Jawbone Radio podcast, the artist who drew the aardvark cartoon, and Randall Munroe from XKCD.com who drew the stickman cartoon.