“Effect” vs. “Affect”

Question

I've noticed that some people use effect and affect interchangeably. What are the differences between these two and when are the proper situations to use each of them?

Answer 1

The noun is usually "effect" -- unless in more formal or literary contexts in which case "affect" as a noun can mean feeling or emotion.

The verb is generally "affect", although "effect" is possible if the meaning is "put into place" or "carry out".

Here are some example sentences:

"His plans had no effect on me."

"His disconsolate eyes brought on a sad affect."

"His plans affected me."

"He effected a plan to change the world."

Answer 2

The "common errors" site mentions 3 different meanings for affect (verb):

• When “affect” is accented on the final syllable (a-FECT), it is usually a verb meaning “have an influence on”:

“The million-dollar donation from the industrialist 
 did not affect my vote against the Clean Air Act.”

• “to make a display of or deliberately cultivate.”
  Occasionally a pretentious person is said to affect an artificial air of sophistication.
  Speaking with a borrowed French accent or ostentatiously wearing a large diamond ear stud might be an affectation.

• “emotion.” (when the word is accented on the first syllable (AFF-ect)).
  In this case the word is used mostly by psychiatrists and social scientists—people who normally know how to spell it.

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When you affect a situation, you have an effect on it.

Less common is a verb meaning “to create” (to cause to come into being):

“I’m trying to effect a change in the way we purchase widgets.”

The Merriam-Webster details:

The verb effect goes beyond mere influence; it refers to actual achievement of a final result.