After twenty-five years of standing before crowds, talking about writing, you live with only two fears: one, at the end of the day you will discover arugula in your teeth; and two, your fly is open. It never occurs to you that someone might ask a question you cannot answer.

I’m standing in front of thirty-five bright new associates at a big East Coast law firm. They have just finished editing a two-page letter, circling typos, spelling errors, grammar slips, usage problems, homonym misuse, and punctuation faux pas—58 total. It’s an exercise in proofreading and paying attention before “sending.” As I review the mistakes and they check their edits, we come to this concocted sentence:

Although the government is not adverse to you entertaining foreign diplomats, you have to no where to draw the line.

Not that difficult to edit: adverse should be averse [See Tip: “Three Words Many People Misuse”]; and no should be know (duh, but these are the mistakes we make when we’re rushed and thinking about other things). When I finish pointing out those two mistakes and move to the next sentence, one of those bright associates raises his hand. “There’s something else wrong.” I have read that sentence five hundred times, and I know those are the only two mistakes. So I think, Arugula? Zipper? The associate says, “You should be your. Your entertaining. A pronoun preceding a gerund should be in its possessive form.” I can feel his confidence in this statement. Now I think, What sort of stupid grammar rule is this? The answer, I discover: one that has caused confusion for almost three centuries.

Quick refresher: Present participles and gerunds both end in -ing—flying, fishing, entertaining—but entertaining, the participle, is a verb, and entertaining, the gerund, is a noun. Therefore, a pronoun preceding a participle must be objective (me, you, her, him, it, us, them)—not averse to you entertaining foreign diplomats—but the pronoun preceding a gerund must be possessive (my, your, her, his, its, our, their)—not averse to your entertaining foreign diplomats.

Fowler would have called what I had written a fused participle, an objective pronoun—you—glued to a present participle—entertaining. Fowler insisted that before an –ing word any good writer would always use the possessive:

Although the government is not averse to you your entertaining. . . .

So the associate was correct: before entertaining, I should have used the possessive pronoun your. But how do we know if the -ing word is a participle or a gerund? They’re the same word! According to the editors at Merriam-Webster’s Concise Dictionary of English Usage, we are not the only ones confused:

“From the middle of the 18th century to the present time, grammarians and other commentators have been baffled by the construction. They cannot parse it, they cannot explain it, they cannot decide whether the possessive is correct. . . .”

Strunk and White—and most other grammarians—agree with Fowler: participles take the objective case; gerunds require the possessive. That’s the easy part. But Strunk and White add, “The difference between a verbal participle and a gerund is not always obvious. . . .” Amen, and if they don’t know, how can we?

1. know your purpose:

What do you think about [me] [my] raising the issue?

If who raises the issue is more important than raising the issue, use the objective pronoun—me. If raising the issue is more important, use the possessive pronoun—my.

2. listen to your ear:

We heard him his yelling “Hallelujah!” in the hallway.

For years I have admired him his skiing.

There is no reasonable expectation of any of them their showing up.

In the first we obviously need the objective—him; in the second we obviously need the possessive—his; and the third makes no sense unless we use the objective—them.

3. if you aren’t sure, opt for the possessive:

The client objected to us our charging for the coffee and donuts.

We truly appreciate you your choosing SilverStar Fidelity.

Treat nouns the same way: If you intend the -ing word to be a participle, use the noun; if you intend it to be a gerund, make the noun possessive.

Are you objecting to the [agent] [agent’s] going with me?

Distinguishing between a present participle and a gerund—and the proper form of word to precede them—is a subtle problem; but good writers get it right, and smart people—like your clients and colleagues—notice.

And now, if you’ll excuse me, I have to go floss my teeth.