1. Affect and effect.
Affect is a verb. Effect is a noun.
If you can use "the" in front of the word you're trying to use, you probably mean to use the noun, "effect". If this is troublesome for you, think of "the sound effect", which is a noun.
2. Lay and lie.
Lay is the action of putting something down. Lie is the action of reclining.
This is a tricky one! I keep in mind that chickens lay eggs, so I can't lay down unless I'm laying down an egg, or some other object. Translate that to all the conjugations, and you'll think twice about saying that something is laying on the floor. Unless it's laying eggs.
3. Passed and past.
Passed is a conjugation of the verb pass. Past relates to a period of time that has already happened, or sometimes a location beyond a defined point.
"Past" can be a noun, adjective, adverb, or preposition. Because "passed" is generally used as a verb, the simplest way to check your usage is to see if your sentence still makes sense using a different verb tense.
-She passed/past a finely dressed gentleman. (Switch test: She passes a finely dressed gentleman. Passed is correct!)
-She brushed passed/past a finely dressed gentleman. (Switch test: She brushed passes a finely dressed gentleman--huh? Passing doesn't work, so it must not be "passed".)
4. Should've and should of.
Should've is the contraction for "should have". Should of is just plain wrong.
5. Then and than.
Then is used in reference to time. Than is used to compare.
There are a lot of ways to use "then", but the word "than" is pretty much only used for comparison. One quick way to check if you're using the wrong word is to check if you're comparing anything; if you aren't, you know you should be using "then".
Flash Round: Mistakes I Make That Leave Me
Gnashing My Teeth Because I Do Know the Difference!
Its and it's
Your and you're
Too, to, and two
Their, they're, and there
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