Use which if the information after it can be taken out of the sentence. Use that if it can't.
The Short Story
If we can take the information that comes after that or which out of the sentence and the sentence still makes sense, then use which with a comma.
Ex: The child ate the ice cream, which was cold.
If we can’t take the information that comes after that or which out of the sentence because the sentence’s meaning would change, then use that without a comma.
Ex: The ice cream that sat in the sun melted.
The Novel
That marks a restrictive clause. A restrictive clause is a part of a sentence that can’t be removed from a sentence because doing so would change the sentence’s meaning.
Ex: The novel that was on the shelf was a page-turner.
In this example, it is important to note that we are specifically talking about the novel on the shelf. Perhaps there are many novels in the room, and we need to distinguish between this novel and the novel on the table that isn’t a page-turner.
Which marks a non-restrictive clause. A non-restrictive clause is a part of a sentence that can be removed from a sentence without changing the sentence’s meaning.
Ex: The typewriter, which was an antique, was a gift from my aunt.
In this example, we can easily take out “which was an antique,” and the sentence’s meaning would not change. The typewriter, whether it was an antique or brand new, would still be a gift from her aunt. Most likely there are not other typewriters around either because we do not have to discern between this typewriter and another.
The use of that and which can make a big difference in sentence meaning. Let’s look at an example:
Ex: The pencils that were sharpened were on the desk.
Remember, using that means that the clause connected to it is essential to the sentence. So, this means that the pencils that were sharpened were on the desk. So, we can conclude that the dull pencils weren’t on the desk.
Ex: The pencils, which were sharpened, were on the desk.
The clause connected to which is not essential for a sentence. This example is pointing out that the pencils on the desk were sharpened. In this example, the fact that the pencils were sharpened is just a side note. It’s not important that the pencils were sharpened or not. Both the sharpened and the dull pencils could be on the desk.
The Examples
“Her findings were contained in a handsome, bound, two-inch-thick report, which Cable got admitted into evidence.”
(Grisham, John. The Runaway Jury. United States: Random House Publishing Group, 2010.)
Here, which is used instead of that because the fact that the report was admitted into evidence is a side note but not a distinguishing feature. Cable probably admitted several other reports along with this one.
“Over the weekend, The New York Times published a report that looked into how police handled the Black Lives Matter protests across the country.”
(The Skimm. “Daily Skimm: Istanbul Convention, Stop Asian Hate, and ‘Zack Snyder's Justice League’.” Daily Skimm. Published March 22, 2021. https://www.theskimm.com/news/2021-03-22-3ExYwXPlV9kpFHsuRRnUI4.)
In this example, the use of that instead of which is very important. We know for a fact that the New York Times publishes several reports a year—reports on the election, taxes, anything really. But this report is different because it specifically talks about how police handled the Black Lives Matter protests across the country. That is why it is important to use that, so we know specifically what report was published. If which was there instead, the focus of the sentence would change from what kind of report was published to a report being published in general.
“Here are 10 SEO methods that will help your site dominate the SERPs in 2021.”
(Dopson, Elise. “10 Effective SEO Techniques to Drive Organic Traffic in 2021.” Single Grain. Accessed March 23, 2021. https://www.singlegrain.com/seo/effective-seo-techniques-that-work/.)
That is used here instead of which to emphasize which kind of SEO methods the sentence is talking about. Here, they are specifically talking about SEO methods that will help your site dominate the SERPs in 2021. If which were used instead, then the sentence would emphasize the SEO methods in general, not the specific kind of SEO methods.
“The Weasley family will be spending a month in Egypt, returning for the start of the school year at Hogwarts, which five of the Weasley children currently attend.”
(Rowling, J.K. Harry Potter and the Prisoner of Azkaban. United Kingdom: Pottermore Publishing, 2015.)
Which is more appropriate in this instance because the clause “five of the Weasley children currently attend” is a sidenote. It does not define Hogwarts in any way. If that had been used, the sentence would be distinguishing between a Hogwarts that the Weasley children did attend and a Hogwarts that the Weasley children did not attend. From this novel, we know that J. K. Rowling refers only to one Hogwarts, so we wouldn’t want to distinguish between other Hogwarts by using that.