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A literary agent has compiled some commonly confused words and commenters have added more.

lie-lay 

Time to lie down for a nap. (verb, present tense)

Yesterday she lay on the grass and daydreamed. (verb, past tense)

       If you are going to use “lay” as present tense, it’s only if you are going to lay something down. The present tense verb “lay” needs to have an object.

affect-effect-effect

Will this post affect the way you write? (verb)

If so, I hope it has a positive effect. (noun)

I’m trying to effect a change in the way writers use grammar. (transitive verb meaning to cause or bring about)