That Is vs. Which Is
This generation, like every one before it and every one to follow, has the dubious pleasure of seeing evolution of language in action. The changes are obvious to careful writers, as they notice with distressingly increasing frequency the erosion of a distinction between words with similar but divergent meanings (for example, anxious versus eager) or a relaxing of a grammatical rule.
One example of the latter that is near or at its tipping point (in this case, the point at which a grammatical error becomes so ubiquitous as to widely be deemed acceptable) is the distinction between restrictive and nonrestrictive, or essential and nonessential, clauses.
Throughout the modern era, at least in American English, careful writers have honored a distinction between the use of “that is” and “which is” and, universally, the insertion or omission of punctuation to begin or bracket a subordinate clause. For example, the sentences “The dog that has a bone is well trained” and “The dog, which has a bone, is well trained” have distinct meanings signaled not only by the difference of a word but also the use of parenthetical punctuation in the latter sentence. For the sake of promoting unambiguous communication, such statements as “The dog which has a bone is well trained” is avoided (though such usage is common in British English).
The phrase “that has a bone” in “The dog that has a bone is well trained” provides essential information: More than one dog is visible to the speaker or writer and the observer or reader, and the person making the statement is providing an additional detail to direct the other person to one dog in particular.
By contrast, “The dog, which has a bone, is well trained” likely refers to a scenario in which only one dog is present. The sentence does not specify whether one or more other dogs are in the vicinity, because such information is irrelevant. The phrase “which has a bone” is providing additional, nonessential information to the base sentence “The dog is well trained.”
Unfortunately, writers often fail to observe the distinction, and even more unfortunately, many of these writers are paid to write; their content is published online and in printed publications that many other people read, and many of these other people see the erroneous content, accept it (it’s published, after all, so it must be correct—right?), and consciously or unconsciously imitate it. Eventually, the tipping point is reached, and (for better or worse) wrong becomes right.
A case in point, with not one but two identical grammatical violations: One writer said of two much-anticipated films, “Before giving us his upcoming Blade Runner sequel that’s shrouded in mystery, director Denis Villeneuve has the sci-fi movie Arrival that is getting incredible word of mouth.” By including the phrases “that’s shrouded in mystery” and “that is getting incredible word of mouth”—with that in place of which and without parenthetical punctuation—the writer creates the impression that the phrases are essential. Evidently, more than one Blade Runner sequel is imminent, and one of them, directed by Denis Villeneuve, is shrouded in mystery. (Presumably, the other is not—or the others are not.) In addition, of two or more movies titled Arrival, one is getting incredible word of mouth. (Presumably, the other is not—or the others are not.)
What the writer should have written is “Before giving us his upcoming Blade Runner sequel, which is shrouded in mystery, director Denis Villeneuve has the sci-fi movie Arrival, which is getting incredible word of mouth.” This sentence describes two unique films, one of which is shrouded in mystery and one of which is getting incredible word of mouth. The phrases that provide those additional details are bracketed by commas to signal that the details are not necessary for one to understand the basic fact that two films directed by the same man are being released in sequence. (The additional information is potentially intriguing but not essential.)
I’ve noticed the lack of distinction between essential and nonessential clauses more and more often over the years, not because I’m more observant than before but because the erosion is more common than it used to be. I know that such evolution is inevitable, but as with any change, one can accept the inevitability yet still resist it. More than that, it is the responsibility of all careful writers to do so.