Chapter 4-1-7: Use "who," "whom," "which," and "that" carefully
Professionals that work with language every day such as writers, editors, and publishers disagree on the rules for using the relative pronouns "who," "which," and "that," in formal writing. This page details the loose standards that are accepted by many people today.
Using "Who" and "Whom"
The pronoun "who" usually refers to people, but may also refer to animals that have names:
Confusion: When to Use "Who" Versus "Whom?"
Every semester there is at least one student that asks me this question. It's a bit tricky and, until you know when to use "who" versus "whom," my best recommendation is simply never to use "whom" at all--always use "who." I say this because most of the time a faux pas in this instance will pass unnoticed by most Anglophones when speaking.
At the same time, that's not a very responsible answer on my part, so I would like to offer a few tips on this topic. The best explanation I've found for this topic is made by The Oatmeal, and I've provided a link to it right here on this page.
If the person is the subject (the person that is doing the action), then use "who." On the other hand, if the person is the object (the receiver of the action), then use "whom."
Now, that explanation is just too tricky for most people, so here are some tips.
Often, a preposition precedes the spot where students would place a "who" or a "whom." Usually, when the word "who" is preceded by a preposition, it becomes "whom" when referring to a person:
Yet another trick from The Oatmeal is to replace the unknown "who-whom" word with either he or him as part of your answer. For this to work, it must be your answer. If you found that the formulation that "sounds right" to your ear uses "he", then use "who." On the other hand, if the sentence sounds more natural with the word "him" as a replacement, use "whom." For example:
Would you state that "He was on the phone" or that "Him was on the phone"? You'd say "He was on the phone." Therefore, you could ask "Who was on the phone?"
Would you state that "I spoke with he on the phone or that "I spoke with him on the phone"? You'd say "I spoke with him on the phone." Therefore, you could ask "With whom did you speak on the phone?" Or you could state "The person with whom I was on the phone did not tell me his name."
Finally, the pronoun "that" refers to animals and things and occasionally to people when they are collective or anonymous:
While the pronoun "who" refers to people, the pronoun "which" refers to animals and things:
Confusion: When to Use "That" Versus "Which?" in Restrictive and Nonrestrictive Clauses
Both of these words are used to provide extra information. If you find knowing the difference about when to use which of these two words confusing, there is some good news: even the best English writers would have difficulty explaining when they are used, and these same writers argue and disagree with each other on what is "correct" grammar usage of these two words.
This explanation expresses the commonly accepted rules between "that" and "which" when they are used in restrictive and non-restrictive clauses. These two words often introduce modifying clauses via restrictive and nonrestrictive or non-defining clauses. A modifying clause always uses that or which after a noun in a sentence. The word "who" is preferred when referring to people.
WHEN TO USE "THAT"
The pronoun that is a restrictive pronoun. Some people call it a defining pronoun. It is part of a restrictive clause. Restrictive clauses do not use commas. Synonyms for the word "restrictive" in this case mean "necessary," "essential," or "required." In other words, the relative pronoun that tells readers an essential piece of information about the antecedent it is referring to. The word "antecedent" means "the thing that came before it." Here, the thing that came before the relative pronoun that is a noun in the form of a place, thing, or concept.
In the above examples, the restrictive clauses are in italics. Observe how they answer important questions. Specific islands, cats, and loves are referenced. Which island specifically? The one that receives the most sun. The fact that it is the one with the most sun is essential to the sentence. Which cat specifically? The one that Henri lost. Which love? The one that I feel for you. Without this information, the reader has no clue which island, cat, or love the author is referring to. Observe that the relative pronoun that was not used to reference people.
WHEN TO USE "WHICH"
The pronoun which is a non-restrictive pronoun. Some people call it a non-defining pronoun. It is part of a non-restrictive clause. Non-restrictive clauses are usually set off (surrounded) by commas. Synonyms for the word "non-restrictive" in this case mean "nonessential," "additional," "extraneous," "optional," or "an aside." In other words, the relative pronoun which provides readers with additional, optional information about the antecedent it is referring to. Remember that the word "antecedent" means "the thing that came before it." Here, the thing that came before the relative pronoun which is a noun in the form of a place, thing, or concept.
In the above examples, the non-restrictive clauses are in italics. Observe how they are offset (surrounded) by commas and how they provide additional, optional information. For example, the sentence "Maui is visited by thousands of tourists every year" is perfectly fine on its own. The fact that that it receives a lot of sun is additional information that is nice to know, but it isn't essential. Furthermore, "My love is eternal," but I don't need to state at whom it is directed. Lastly, all that most people need to know is that "I found a cat." They don't necessarily need to know that poor Henri lost it. Notice in this last sentence that the sentence can end with the non-restrictive clause. They don't have to be sandwiched between two clauses. Observe that the relative pronoun which was not used to reference people.
A trick that you can use to test if the clauses is restrictive or non-restrictive is to try and put the clause in parentheses. If it works well in parentheses, it is a non-restrictive clause and you should use "which." For example, "My love (which is reserved only for you) is eternal." It works! So, this is a non-restrictive clause and it should start with "which."
WHAT ABOUT PEOPLE?
This wasn't really an explanation about restrictive and non-restrictive clauses. It was about moments when authors confuse "that" with "which." Briefly, since you're here, the word "that" is used when referring to people in restrictive clauses and the word "who," "whose," and "whom" are used for non-restrictive clauses:
Observe that the first sentence is a restrictive clause. Which people are specifically being referred to? The ones that buy their groceries locally. Do you see how if the restrictive clause were to be removed and we were left with "People are more conscious about the environment" is a completely different idea? On the other hand, the second sentence is a non-restrictive clause. Observe how it is offset by commas and how "who just arrived in town" could easily be removed from the sentence without changing its meaning. The fact that Morgan just arrived in town is bonus, optional information that didn't have to be mentioned.