Affect* is a verb, and can have a couple of different meanings. The least common use of the two meanings is “to pretend or assume (as in, “take on”).
“I affect a terrible British accent, but my friend can swear like a drunken Scot.”
“Nervous before every board meeting, Shondra affects a calm she does not feel.”
The second, most common use of the verb is “to influence, act on, or produce an effect.:”
“The loss of the homecoming game affected the team’s morale so badly that they never won another game that year.”
“My hope is that my election to the committee will affect some change in how that body is run.”
Effect** is almost always used as a noun meaning “a consequence or result” and is used with an article:
”The effect of the sale is that the company will have to lay off 20% of its work force.”
“My joke at the funeral did not have the effect I was hoping for.”
As Kizz pointed out in one of the comments for the last Grammar Wednesday, a lot of these questions boil down to elocution. In this case, though, a little mumbling can be your friend: it’s hard to tell the difference between “affect” and “effect” when speaking. When you’re writing, though, choose the right word by trying to replace the word with whatever tense of “influence” you need. If “influence” works, use “affect.” If “result” works, use effect. For example:
“The loss of the homecoming game influenced the team’s moral…” works (but result doesn’t)
“My joke at the funeral did not have the influence I wanted.” doesn’t. (but “result” does)
You can also figure out if you’re using the right word by putting “an” or “the” in front of the word in question. Effect takes an article; affect does not.
Affect / Action
Effect / Result
*Affect also has a noun form, but it used almost exclusively in psychological contexts (and, I’m fairly sure, some Shelley poetry) and, as such, most of us don’t bother to waste valuable brain cells in learning the meaning. If you have brain cells to spare, however, the definition of the noun form of affect is “a feeling or emotion.”
**Effect has a verb form, too; but, again, it’s not at all common. The definition, according to my Webster’s dictionary, is “to bring about, accomplish, or make happen” and the example they give me is “The change to automation was effected last spring.” In all my years studying English and writing for college, I’m not sure I’ve ever used effect as a verb.