Fortunately, here is a simple guideline for modern writers:
Use “that” at the beginning of a phrase that is essential to the sentence.
Use “which” at the beginning of a non-essential phrase that won’t be missed if you decide to delete it.
The essential phrase—beginning with “that”—is known as a “restrictive clause,” so named because the writer is restricted (required) to include the information in order to convey the sentence’s full meaning.
The non-essential phrase—beginning with “which”—is called an “appositive phrase,” so named because the words in the phrase are in apposition to (added to) the rest of the sentence.
The following two sentences are almost identical—except for “that” or “which” and two commas—but the difference in meaning is huge.
My Mercedes-Benz that my uncle owned for 15 years is parked in his garage.
My Mercedes-Benz, which my uncle owned for 15 years, is parked in his garage.
The first sentence suggests that the writer owns more than one Mercedes-Benz and that the one once owned by an uncle is still parked in the uncle’s garage. All of the words in this restrictive clause are essential in order to distinguish that Mercedes-Benz from others the writer owns.
The second sentence indicates that the writer owns one and only one Mercedes-Benz and it’s parked in an uncle’s garage. The appositive phrase also informs that the uncle once owned the car for 15 years—a nice little detail but not essential to the main message.
Because the writer owns only one Mercedes-Benz, the sentence could simply be:
My Mercedes-Benz is parked in my uncle’s garage.
Because an appositive phrase is not essential, it is separated from the rest of the sentence by commas.
Some appositives, those not identified by “which,” can be harder to recognize. For example:
George Washington, the first President of the United States, did not sign the Declaration of Independence.
Is the phrase “the first President of the United States” essential? No. Does the phrase have value? Yes; it points out a historical detail.
The key to properly using and punctuating “which” and “that” is an exercise in nuance, accuracy and clarity. Read the sentence from the readers’ point of view and you will know what to do.
On a road trip across the United States in 2012, I camped one night on the canyon’s south rim a mile above the Colorado River. As I captured this photo image, I whispered, “I sure would like to raft that river.” And then I stated, “Why not?”
Determined to fulfill that desire, I drove a few miles to a general store I had passed earlier. There, I used the store’s pay phone—my cell phone couldn’t pick up a signal in that wilderness—and called four rafting outfitters. One of them, Arizona River Runners, had space for one person—on a raft that was leaving from the upriver put-in point in two days. I booked my seat and drove there the next day.
Others in our rafting party were amazed at my good fortune, having scheduled their trips months in advance. I smiled, knowing that we can achieve what we ask when we are bold enough to act.
Photo by Robert M Weir, July 2012, South Rim of the Grand Canyon, Arizona, USA.