After my previous post on two commonly used relative pronouns, this topic seems appropriate. And while this one seems intuitive for me, I often see evidence that it isn't the case for other writers. So when should you use the relative pronoun who rather than that? For some writers and editors, the rule is straightforward enough that its “misuse” is a pet peeve, but others play fast and loose with it. But like so many other grammar rules, it's not always black-and-white, and there are a few exceptions.

Consider the sentences below:

Kate went shopping with her friend that needed a new suit.

David mowed the law for his neighbor that was out of town.

So what's wrong with them? Grammatically, they are indeed correct. That can, in fact, refer to people, especially if the relative clause is restrictive. However, that doesn't mean that it's the best relative pronoun to use in this context. Compare the sentences above to the following:

Kate went shopping with her friend who needed a new suit.

David mowed the lawn for his neighbor who was out of town.

Even those who aren't strict about this rule will admit that they read much better. Many style books would favor these versions of the sentences as well. Keep the following guidelines in mind as you determine whether to use that or who.

When to Use Who
As a relative pronoun, who is fairly limited. Who (as well as its inflections whose and whom) can only be used to refer to people or entities equated with people (like deities and occasionally pets). It should not be used when referring to things or animals.

I followed the girl who was running down the street.

When to Use That
That can refer to animals, things, and people and should be used when the clause is restrictive. While that can be used to refer to human beings, it is not the preference. The following is a correct use of that:

Bill found the car that he wanted.

So if you have the option of that or who in certain contexts, what's the problem? You'd probably get different answers depending on who you ask. But for me, using that in reference to people is similar to referring to someone using the pronoun it. You wouldn't do that, would you? It would make that person seem less human.

Exception to the Rule
As you've come to expect with grammar rules, there is an exception . While you should use who in reference to people whenever possible, that can (and should) be used when the sentence has more than one relative clause and who has already been used. This will help avoid awkwardness and repetition.

That is the woman who shared her apartment with the man that took her money.

Keep these guidelines in mind when you need to use relative pronouns.

We all play different roles in various aspects of our lives. Any one of us can go through the day wearing different hats (writer, editor, spouse, cook, home maintenance, some of us are parents, etc.). And like people, words play different roles depending on their context in a sentence. While this seems obvious, it can cause confusion and misuse of terms, which brings about ambiguity or potential misreading.

For instance, the terms that and which are prone to misuse when used as relative pronouns introducing adjective clauses. Keeping a few simple rules in mind will help eliminate confusion so you can use these words correctly. Take a look at the following examples:

The bedrooms that we painted during the summer look cheerful and bright.

The bedrooms, which we painted during the summer, look cheerful and bright.

Both of these sentences describe the bedrooms, but what the first sentence tells you is completely different from what you learn from the second one. In the first sentence, you see multiple bedrooms but only the recently painted ones appear cheerful and bright, but the second one says that all the bedrooms look cheerful and bright and mentions that they've been painted.

The Restrictive Clause
The adjective clause in the first example sentence above (that we painted during the summer) is called a restrictive clause because it limits the meaning of the nouns it modifies. The restrictive clause introduces information that is essential to understanding the meaning of the sentence, and that is the relative pronoun normally used to introduce this clause.

The Nonrestrictive Clause
The adjective clause in the second example sentence (which we painted during the summer) is called a nonrestrictive clause because it modifies the noun but does not necessarily limit its meaning. All of the bedrooms look cheerful and were painted. The nonrestrictive clause introduces information that could be helpful but is not essential to the understand the sentence, and which is the relative pronoun usually used to introduce the nonrestrictive clause. And as you can see from the example, the nonrestrictive clause should be separated from the rest of the sentence with commas.

Exception to the Rule
Like a number of grammatical rules in English as well as other languages, this one has an exception, but it should only be used when a sentence has more than one dependent clause or when that has been used in another role. Consider the following example:

That concept, which has been discussed thoroughly, no longer needs to be addressed.

If this, that, these, or those has already been used as an adjective or if that has introduced the first clause, use which to open the next one, whether the information is essential or not.