Must Of Been a Mistake

I received an email from my website form about people using “of” where they should use “have” in their writing. It was really more of a comment than a question. I wasn’t going to address it because I didn’t know where I’d talk about it, but then I started this newsletter, and here we are.
The Correction

We’ve all probably seen something this, especially in casual American English, like a tweet or a Reddit comment:

He must of known what would happen.

Let’s break down what’s grammatically going on. “Of” is a preposition, and like most prepositions, it shows a relationship. It’s also an extremely common word in English; it really gets a workout. It can mean about: stories of adventure. It can mean belonging to a group: one of the people. It can be a possessive: the gowns of Princess Diana. It can mean direction or positioning: east of Eden. There are a couple dozen ways to use “of” in English.

None of those ways, however, are as a verb. “Have” is a verb, and it too plays a lot of roles in English. It most often means possession: I have three cats. It can also mean obligation: She has to leave now if she wants to make it on time. It can mean to give birth: They had a baby yesterday. It can mean to partake: He has a drink after dinner.

But that’s “have” all on its own. It can also be an auxiliary verb to form what are called the perfect tenses: present perfect, past perfect, and future perfect. These tenses add some nuance to the sense of time that all verbs convey. In fiction, we most often see past and past perfect tenses. So a character does something, which is written in past tense, but some other action came before, which requires past perfect. Here’s an example:

She rode her bike to school. Luckily, she had filled her flat tires the night before, so the ride was smooth.

“Rode” is past tense; “had filled” is past perfect because it happened before she rode the bike.

All right, so we have “of” the preposition and “have” the verb. How do these get confused?

Easy. We say words out loud.

Say we add another auxiliary verb like “must” to a sentence to show that someone was commanded to do something, or compelled to do it, or is determined to do it, or any other number of ways we use “must” to intensify a verb.

Let’s say another character saw our girl riding her bike to school and gliding smoothly along the rough pavement:

She must have filled her tires the night before.

The “must” here shows that the observer is inferring what the rider did to achieve such a sweet ride. If you say it aloud, or hear it in your head, it’s obvious how the error slips in. “Must have” and “must of” sound alike to most Americans, especially when we’re speaking quickly.
The Reality

Here’s a secret about editors that’s not at all a secret because we’ll tell everyone everywhere at the drop of a hat: We know all this word nerd stuff, but we also know that it doesn’t get applied equally everywhere.

If I encountered “must of” in a novel, especially if it was in dialog, there’s a chance I would leave it as-is. Say a character is trying very hard to seem like a smart, together kind of guy. He’s dressed in a suit, but it doesn’t quite fit right. He’s got a big office, but it’s in a seedy part of town. And sometimes, the way he talks betrays him. “He must of known what would happen,” he says to a visitor in his too-bright office. This tells the reader a lot about the character, this “must of.” Does he know it’s “must have”? Does he not? Does his visitor speak the same way, or does she notice this slip? These are the kinds of choices that authors make to create fully embodied characters.

In nonfiction or academic pieces, it’s “must have” all the way, no questions. But in fiction, depending on the authors voice and the story’s style, and depending on what I know of the characters, I might merely flag a “must of” with a comment in the margin. I’d ask the author if this is an intentional choice to show something about the character or if it was a simple mistake that needs to be corrected to “must have.”

If you want to make that “must of” choice, you really have to mean it. Microsoft Word (and likely other autocorrect functions) changes it to “must have” by default, which has made writing this essay a challenge.