1. When it is appropriate to use When to use Who instead of that
That vs. Which (and Who)
When it is appropriate to use
that VS. which
When to use Who instead of that
For more information on deciding between that vs. which, please visit:
Also visit:
For additional instruction on this topic, refer to the That vs. Which section of the Grammar module.
For a short exercise on “that” vs. “which” vs. “who”, please visit:

2. That
That should be used to introduce a restrictive clause. A restrictive clause is one that is essential to the meaning of a sentence. If a restrictive clause is removed, the meaning of the sentence will change.
Correct: The mice that were given the experimental drug were drowsy.
This sentence implies that some mice were given the drug and others were not; only those that were given the drug were drowsy.
The part of the sentence that is underlined is called the restrictive clause.
There are multiple mice (as implied by the use of the plural, mice, rather than a single, mouse). The point of the sentence is to indicate clearly which of the mice were drowsy by providing what is sometimes called essential information. Calling the restrictive clause essential does not imply that it is important; it implies that it is necessary to clarify which mice were drowsy because not all of them were. This sentence indicates that only those given the experimental drug were drowsy, not the others (i.e., not those that were not given the experimental drug).
As you will see on the next slide, if you used which (preceded by a comma) instead of that, you would imply that all the mice were drowsy.

3. Which
Which should be used to introduce a nonrestrictive clause. A nonrestrictive clause can be left out and the meaning of the rest of the sentence will not be changed.
Correct: The mice, which were given the experimental drug, were drowsy.
This sentence implies that all the mice were given the drug, and all were drowsy.
The part of the sentence that is underlined is called the nonrestrictive clause.
There are multiple mice (as implied by the use of the plural, mice, rather than a single mouse). The point of the sentence is to provide additional (nonessential) information about the mice. This sentence indicates that all the mice were drowsy, and that the mice were given the drug. The sense in which the information about the drug is nonessential is that it is not required to clarify which mice were drowsy. Nonessential does not imply that the information about the drug is not important.
A nonrestrictive clause is always placed between commas. Correct: The mice, which are white, are given the placebo. Incorrect: The mice which are white are given the placebo.

4. That vs. Which Changes the Meaning
The mice that were white were given the placebo.
The mice, which were white, were given the placebo.
The dogs that took the obedience class would stay on command.
The dogs, which took the obedience class, would stay on command.
In the sentences using that, only the mice or dogs described in the restrictive (a.k.a., essential) clause experienced the relevant event or behavior (i.e., placebo or stay).
In the sentences using which, all the mice or dogs experienced the relevant event or behavior, and in addition, the mice were all white and the dogs all took the obedience class.

5. Maximize Clarity
The dogs that ate the dark chocolate chips had to have their stomachs pumped, but the one that ate the butterscotch chips did not.
Only some dogs had to have their stomachs pumped; those were the ones that ate dark chocolate, which is poisonous to dogs in pretty small amounts. This part of the sentence implies that there were other dogs that did not have to have their stomachs pumped, and the second clause tells us which ones were so fortunate. Butterscotch chips are not toxic to dogs.

6. Who
Use that or which when referring to an object, but use who when referring to a person.
The people who ate the pizza threw up.
The people, who ate the pizza, threw up.
The dogs that ate the pizza threw up.
The dogs, which ate the pizza, threw up.
The people who ate the pizza threw up. This indicates that only the people who ate the pizza threw up. It implies that there were other people who did not throw up (the ones who did not eat the pizza).
The people, who ate the pizza, threw up. This indicates that everyone included in group described by the people threw up, and they were also the ones who ate the pizza. Here, eating the pizza is nonessential information because it does not restrict the group of people who threw up, whereas it is essential information in the previous sentence.
The dogs that ate the pizza threw up. This indicates that only the dogs that ate the pizza threw up. It implies that there were other dogs that did not throw up (the ones that did not eat the pizza).
The dogs, which ate the pizza, threw up. This indicates that all the dogs threw up, and all the dogs ate the pizza. Here, eating the pizza is nonessential information because it does not restrict the group of dogs who threw up, whereas it is essential information in the previous sentence.