When to use which or that is one of the most grammatically confusing grammar lessons ever taught. The fact that the two words are considered practically interchangeable in modern English does not make learning the distinction between the two much easier. You could sit through four years of English classes and still not fully understand when to use "which" or "that." Hopefully, the illustrations below will help you.
When to Use Which or That
Restrictive and Non-Restrictive Elements
The elements up for illustration are not earth, wind or fire. Let's talk sentence elements. They are restrictive and non-restrictive elements.
A restrictive element is a word, phrase or a clause that manages to limit the meaning of the sentence element that it modifies. When a restrictive element is not included then the entire meaning of the sentence will change.
The non-restrictive element is a word, phrase or a clause that provides excess information about the beginning of a sentence without restricting the meaning of that part of the sentence.
Keep these two definitions in the back of your mind because they will be crucial in understanding when to use "which" or "that."
Using Which vs. That
"That" Is Restrictive
The word "that" is considered to be a restrictive element of any sentence that it may be used in.
Look again at the definitions above - a restrictive element limits the meaning of the sentence element that it modifies. For example, in the sentence, "Baby foods that contain soybeans are best," the restrictive element of the sentence are the words "that contain." These words restrict the type of baby food that is being discussed.
In effect, without the words "that contain" the whole sentence meaning would be altered. In fact, there would be no restrictive element of the baby food. Instead the sentence would imply that all baby food is best.
"Which" Is Non-Restrictive
Let's examine the notion of the non-restrictive element. Remember, a non-restrictive element is a word, phrase or a clause within a sentence that only serves as additional information - an info item so to speak. Should a non-restrictive item be left out of the sentence, it will not change the meaning of the sentence whatsoever.
You can usually recognize a non-restrictive element because it is surrounded by commas or parentheses. Here is an example of how a non-restrictive element using the word "which" should be used: "Soybean baby foods, which are Sally's favorite, work well for her diet."
Can you see the difference between how the word "that" (a restrictive element) and the word "which" (a non-restrictive element) work in a sentence? If you look at the illustration for a non-restrictive element (or the word "which") you can see how if you removed the phrase "which are Sally's favorite" the meaning of the sentence does not change. The only thing the phrase "which are Sally's favorite" does is provide additional information.
It is important, when reviewing how to use "which" or "that" in a sentence that you take your time and let the lesson sink in. If you read the sentence too fast, you may miss it. When it comes to these words it helps to fully understand about restrictive and non-restrictive elements and the role that they play in sentence structure. Misunderstanding that could lead you to use the words "which" and "that" in the wrong context.