Putting on my anal-retentive English teacher hat

I try very hard not to be a prissy know-it-all (really I do), but tonight I'm putting on my anal-retentive-English-teacher hat and giving my readers some lessons in usage and punctuation. In the last two days, I've seen several well-educated people make 4th-grade errors, and I just can't keep quiet any longer. So here we go:

Don't abuse the apostrophe!

Use an apostrophe before an "s" in a contraction, to replace the missing letter(s). Use an apostrophe for most possessives, but not possessive pronouns. Do not use an apostrophe for plurals!!!

• It's a wonderful life... till the zombies come. "It's" is short for "It is," so you use the apostrophe in place of the missing letter.
  
• The zombies chased Karen's boyfriend into an alley and ate his brain. The apostrophe in "Karen's" indicates a possessive; the soon-to-be-brainless boyfriend belongs to Karen.
  
• The zombie devoured its prey. "Its" is a possessive pronoun. That's English teacher jargon, but it means that you don't need the apostrophe, because "its" is possessive enough all by itself (kind of like that crazy guy you dated in college who followed you around and read your e-mail and stalked you... but I digress).
  
• The check's in the mail. In this classic lie, "check's" is a contraction for "check is."
  
• Now pay close attention to this one, lest I come through your computer and slap you silly for getting it wrong: No checks accepted. "Checks" is the plural of "check" (that means more than one check for those of you who, like me, graduated from Tracy High), NOT a contraction, so you do NOT need an apostrophe! Got it? Now, back to the zombies...
  
• Here's a tricky one: The zombies crashed the Smiths' party, leaving behind twelve brainless corpses and a bloodstained tablecloth. Here we have that bane of the punctuation student's existence, the brainless corpse... uh, I mean the plural possessive. Note that the apostrophe belongs at the end of "Smiths'" and is there only because "Smiths'" is a possessive. If it's only plural, you don't need the apostrophe: The party was crashed by three Smiths, all of whom were zombies.
  

If you can master these basic concepts, you'll be ahead of about 80% of the US population, including quite a few people with graduate degrees. Apostrophe abuse has become so common that someone in the U.K. formed a society to prevent it: the Apostrophe Protection Society. Please do your part to end these senseless crimes against innocent punctuation marks.

A few usage lessons

Now that I've bored everyone silly with my dissertation on apostrophes, I'll keep this part brief. If you want a thorough list of English usage errors, see Paul Brian's Common Errors in English Usage page. I bow in his general direction.

Here are two errors I've encountered recently:

• affect vs. effect: "affect" is a verb; "effect" is a noun.
• The sight of all those zombies eating my co-workers affected me deeply.
• Though the massacre was a tragedy, one positive effect of the zombie slaughter was a significant reduction in the number of meetings I have to attend.
• And, for you psychiatrists and psychologists in the crowd, "affect" can also be a noun when referring to one's mood or emotional state: Many of the people who witnessed the zombie attack exhibited flat affect for many weeks afterward... or maybe they had become zombies. It's hard to tell with some people.
  

• principal vs. principle: "Principal" can be either an adjective or a noun. In either case, it refers to the major, important, or high-ranking part of something:
• The principal reason Harold was eaten by zombies was that he tripped over an air pocket while running down a dead-end hallway.
• After he became a zombie, the principal feasted on the brains of students and teachers alike.
• "Principle," on the other hand, refers to a matter of law or belief: A zombie's guiding principle is the pursuit of brains.
  

As I encounter more usage errors, I'll post them here, relating them to zombies whenever possible.