"Effect" vs. "Affect"

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."

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.

[…]

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.”

Merriam-Webster details:

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

The rule that mostly works it this: affect = verb, think "a" for action, wheras effect = noun, the result of the action

to remember: "a" comes before "e" in the alphabet, and you must affect something to cause an effect

Here’s a quick informal technique for you: If it is not easy for you to remember that that the word “affect” is most commonly used as a verb while “effect” is usually used as a noun, then label this confusion as “aven.” It sounds like amen. The “av” in aven should make you recall affect as verb and the “en” is effect as noun.

Source: A Snappy Guide to Differentiating “Affect” and “Effect”