What are the rules for the use of "that" and "which"?

Who's asking: Mary Maschino, Gardiner, ME

At some point, grammar rules fall into such complete neglect that they might as well not even exist anymore. I often wonder whether this has happened to "that" vs. "which," and I wonder even more about "that" vs. "who."

Still, my whole life's been about lost causes and hopeless cases, so here's the rule. (Note: just to confuse things, this is the rule for American English; the rule for British English is almost the opposite. If you're reading this in the U.K., disregard the next couple of paragraphs.)

Using "that" instead of "who" is an error that irritates me more, so let's take care of that first: "that" refers to animals and inanimate objects. "Who" refers to people. If you write "the man that shot me," instead of "the man who shot me," you are not only demonstrating your ignorance, you are expressing a disregard of other people's humanity that borders on sociopathy. (Yeah, I feel strongly about this.)

"That" introduces an essential clause that is necessary to identify whatever you're describing: "This is the gun that shot me."

"Which" introduces a nonessential clause that merely provides additional information about something you've already identified. "The gun, which the killer had purchased the day before, still shone with its original factory oil."

A good rule of thumb, if you're confused, is to look at the punctuation. "Which" almost always needs a comma in front of it; "that" rarely does.

First Five Random Songs off the iPod Shuffle this morning:

“Pain (Makes You Beautiful),” the Judybats. Title track off a great album. The Judybats are a Knoxville-based band that was very popular in the DC area in the early to mid-1990s... they’re still playing, but I don’t think they have a record deal anymore. They put on an excellent show, if you ever get a chance to see them live.

“Amazing Journey/Sparks,” The Who. From Tommy; like several tracks in this rock opera, it doesn’t stand particularly well by itself.

“Rainy Days and Mondays,” Cracker. Another great cover from If I Were a Carpenter. If I ever wanted to send someone spiraling into depression, I’d make him a CD that included this song, along with Sinead O’Connor’s cover of “Gloomy Sunday,” Beck’s “Guess I’m Doing Fine,” and a few other choice cuts. Better hit the fast-forward...

“White Limousine,” Duncan Sheik. Duncan Sheik is trying hard to be this generation’s Nick Drake (without the suicidal mood swings, I hope). It’s not as distinctive a sound – or maybe Drake’s influence is too pervasive on Sheik and his contemporaries – but I still like this album a lot.

“Where I Go,” Natalie Merchant. One of the less memorable tracks on this album (Tigerlily).