Of all the grammar errors I see, confusing who and whom is one of the most common. I suspect that, once again, it comes from teachers correcting students in school. Now, I'm not saying teachers shouldn't correct student errors, but often the corrections, at least in my case, were in favor of whom with no explanation why the who was wrong. Who is more intuitive, but when many writers are unsure which one to use, they default to one or the other depending on the errors made early on. And they may be correct at times. However, isn't it better to know how to handle these small grammatical details so you can focus on more important issues in your writing?

Luckily, figuring out whether to use who or whom is relatively simple.

Who or Whom?
When considering whether to use who or whom, you need to know if you need a subject or object pronoun. I know this can be tricky. After all, the terms in we're discussing appear in questions or inverted sentences and dependent clauses. So telling you to use who when referring to the subject of a sentence and whom for the object may help, but it still may be confusing. That's fine, because there's a quick way to figure it out.

Using Who
The easiest way to decide whether you need who or whom is to substitute who/whom for another pronoun, like he/him (or she/her). So let's try this with the following sentences:

[Who/Whom] walked through the door?

He tried to find the woman [who/whom] owned the house.

Now, you wouldn't reconstruct the first sentence as “Him walked through the door,” would you? With the second sentence, you'll want to isolate the dependent clause. That will clarify: : “[she/her] owned the house.” So the sentences should read:

Who walked through the door?

He tried to find the woman who owned the house.

Using Whom
The examples above illustrated when to use who. But what about whom? Let's look at another example:

[Who/whom] did you have lunch with?

This one's a bit trickier, but taking a moment to rewrite it as a declarative sentence will help:

You had lunch with [he/him].

Does that help? So the original question should read:

Whom did you have lunch with?

Remember the M
I initially recommended to replace who/whom with he/him for a few reasons. First, it makes deciding whether you need a subject or an object easier. Also, simplistic as this sounds, the m in him should be an easy reminder to use whom. If you don't have an m, use who.

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.