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?
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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 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
The "common errors" site mentions 3 different meanings for affect (verb):
The Merriam-Webster details: