As an editor, you begin to notice common mistakes that writers make and let me assure you, after decades of editing experience, I'm certain of one thing: the "which vs. that" struggle is real! And if you're unsure of which one to use, you're not alone.
So, let's take a look at the process of determining whether to use "which" or "that" to introduce a clause, which is really just a matter of looking at the text that follows either. But before we do, let's do a quick grammar review of restrictive clauses and nonrestrictive clauses, just to refresh your memory on what both are and how they are used in a sentence.
Restrictive clauses
This writing resource defines restrictive clauses like this:
A restrictive modifying clause (or essential clause) is an adjective clause that is essential to the meaning of a sentence because it limits the thing it refers to. The meaning of the sentence would change if the clause were deleted. Because restrictive clauses are essential, they are not set off by commas.
Nonrestrictive clauses
The same source defines nonrestrictive clauses like this:
A nonrestrictive modifying clause (or nonessential clause) is an adjective clause that adds extra or nonessential information to a sentence. The meaning of the sentence would not change if the clause were to be omitted. Nonrestrictive modifying clauses are usually set off by commas.
So, what does this have to do with "which" and "that"?
The reason we've reviewed the definition of restrictive and nonrestrictive clauses is simple. If the words that follow "which" or "that" are a restrictive clause, you need to use "that". If they are a nonrestrictive clause, "which" is the correct choice.
Let's see how that works with a few examples:
The car that hit me yesterday was driven by an unlicensed driver.
In the above sentence, "that" is the correct choice because "that hit me yesterday" is a clause that is necessary to the meaning of the sentence. If we don't know the information that the clause gives us—"that hit me yesterday"—then important details will be missing in the sentence and the sentence will change in meaning. Without that clause, the sentence would just be "The car was driven by an unlicensed driver."
Well…okay, but which car? And why are we bringing it up? See how it is important information?
Now let's look at another example:
That new restaurant, which I didn't realize was there, is now my favorite place to eat on South Main.
In the above example, "which I didn't realize was there" is a nonrestrictive clause because it could be removed from the sentence and the sentence would retain its meaning. "That new restaurant is now my favorite place to eat on South Main."
Now what about the commas?
Knowing whether to use "which" or "that" will also help you know the correct way to punctuate the clause, in most cases. When you use "which", the clause should be set off by commas—meaning that there should be a comma before "which" and another comma at the end of the clause. Doing this also helps visualize whether the clause can be removed without changing the meaning of the sentence.
Alternately, with a restrictive clause beginning with "that", commas are not needed to set off the clause.