Mable Scott asks, What is the rule for using affect or effect?

Affect

Affect is typically used as a verb meaning “to influence.”
Example: “The rain affected the skier’s performance.”

It can also mean “to put on a false show of; simulate.”
Example: “Frank often affected a British accent because he thought it made him sound smart.”

It is less commonly used as a noun to mean “feeling or emotion, especially as manifested by facial expression or body language.” As a noun, affect is pronounced with stress on the first syllable rather than the second (also, with a short “a” sound rather than a schwa).
Example: “It was difficult to tell whether Harold’s flattened affect was a result of his mental disease or the medication he took to treat it.”

Effect

Effect is typically used as a noun meaning (1) “something that follows a cause,” or (2) “an influence,” or (3) “a distinctive impression.”
Examples:

1. Politicians are arguing about what the effects of Social Security reform will be.
2. That movie had a profound effect on the way I think about Rwanda.
3. The blue tint gives the painting the effect of being cold and austere.

It can also be used as a verb meaning “to cause to come into being.”
Example: “The sweeping reforms effected a dramatic change in the way citizens viewed their civil liberties.”

The distinction between affect as a verb and effect as a verb can be quite subtle, so beware. Thanks to Mable for the great question.