I spend a lot of time reading message boards, and comments at the foot of news stories. I find some very consistent word usage errors, and some of them drive me nuts. Some are so simple that I haven't made that mistake since Grade Three, and some, I still have to stop and think about.
Let's start with some of the easy ones.
"There" refers to a place, as in, "here or there."
"Their" is a possessive, as in, "their car."
"They're" is a contraction of "they are," as in, "They're going to the store."
We use the apostrophe to take the place of the missing letter in a contraction, like this: "It's (it is) a long way to the store, but Dad's (Dad is) going to walk." We also use it to indicate a possessive, with a noun. "The dog's bone." "The cat's pajamas." It is actually also a contraction, because, back in the day, the phrase would have been, "the dog, his bone," and "the cat, his pajamas." If you don't believe me, read some old book, like "Canterbury Tales" or "Le Mort d'Arthur."
A trickier one is "principal" and "principle." "Principal" refers to the main guy, like a school principal or, as an adjective, "the principal (main) actor in the play." A "principle" is a fundamental law or assumption, as in, "a man of principle."
Another is "effect" and "affect." "Cooking carrots affects the cellulose content, making them easier to digest." As a noun, "affect" is sometimes used to describe behaviour; "he has a flat affect." "Effect" is usually caused by something, hence our phrase, "cause and effect." Like most English words, it has several meanings, but we usually use it as something that is a result of something else. "The effect of the makeup was to make him look older."
This brings me to "accept" and "except." "Accept" is a verb. It mainly means to willingly take something offered. "She will accept the award." "Except" is a conjunction, and means to exclude something. "Everyone except Mary went out."
One mistake that seems to be gaining in popularity is the use of "allude" for "elude."
"Allude" requires "to," just like "refer." Just as you can "refer to" something, so can you "allude to" something. "Allude" is a more general term than "refer," You might allude to dramatic themes, but you can refer to "Hamlet" to back up your allusion. "Elude" has an entirely different meaning--to avoid or escape, as in "He eluded the posse."
...I am being distracted by a television biography of Catherine the Great, so I will take up this fascinating subject again at a later time.