As I was driving home from dropping Zenella off, I saw a billboard that made me think again about gerunds (well, to be honest, made me think about blogging about gerunds).
The sign said: "Remember your son rebelling with a motorbike. Retirement is payback time." This is a clear example of a gerund's being used without its being possessed. (Anyone who loved that self-reference and hasn't read Goedel, Escher, Bach is directed to do so forthwith.) In the following, I should note, I'm using "correct" in a narrow sense. I do not consider the sentence I am discussing incorrect in a strict sense, because this usage is probably more common than the "correct" one. I mean only that it is incorrect for someone who wishes to use gerunds strictly correctly.
Why is it incorrect? How can you tell? You have to ask yourself a simple question. What are you being asked to remember? Your son, right? Wrong. You are being asked to remember the rebelling your son did, specifically that he bought a motorbike to rebel. If you are struggling with this, ask yourself what circumstances would need to exist for you to be commanded to remember your son. Do those circumstances exist for most people who are reading that billboard? No. Furthermore, the second sentence confirms that what we were talking about in the first sentence was the rebelling, not the son.
So this sentence should read: "Remember your son's rebelling with a motorbike." That would gladden Fowler's heart.
Generally, experience and thought will show you what is a gerund and what just looks like one -- in most cases, a participle. It's important to be clear what each is. A gerund is a noun, which describes the action of a verb. "Rebelling" in this case means "the act of rebelling". Whenever you are talking about the act as a thing, you are using a gerund. Participles are adjectives. They describe the noun that they are attached or adjacent to. "Remember your rebelling son" and "Remember your son's rebelling" are clearly different ideas, describing different things. In the former, the son, clearly; in the latter, the act of rebelling, clearly.
Look at this sentence: "I am preparing for you/your coming". This is an easy one to stumble over. Surely, you might say, you are preparing for me, so it's "I am preparing for you coming". But if so, what is "coming"? What is its part in this sentence? Why, if you are preparing for me, did you not simply say "I am preparing for you"? Well, the reason you didn't is that what you are preparing for is my arrival. See how much easier that sentence is with a word that is unambiguously a noun. A good test for gerunds is to replace them with a similar word that is unambiguously a noun, or even that is not so similar but you feel it works the same: "Remember your son's comeback with a motorbike" is clearly enough the same sort of sentence as the one we're considering.
This would all make things easy if it were not that you can use participles after nouns in some cases. Look at these sentences:
He is not aware of the tax applying to his investment.
He is not aware of the tax's applying to his investment.
Both are perfectly correct, but they do not have the same sense. Sets of sentences like this are why some are careful with gerunds. The distinction in sense is lost if you are not.
The first sentence means he is not aware of the tax that applies to his investment. The tax has not been previously defined. You could in principle replace "applying" with "that applies". "That" clauses -- defining clauses -- function similarly to adjectives. They "define" the noun, just as an adjective does. So it should not be surprising that they can interchange with adjectives. "Which man did you talk to? The man looking like Elvis." This construction is often used in speech but is not incorrect in writing. You would normally write "The man who/that looks like Elvis", which is more euphonous.
The second sentence means that he is not aware that a tax that has already been mentioned applies to his investment. You cannot replace "the tax's applying" with "the tax that applies" for a simple reason: the gerund is not defining the tax. Indeed, in a sense, the tax is defining the gerund!
Smarter monkeys among us will have grasped that there is a simple test you can do to decide whether you have a gerund, which will require possessing, or a participle, which does not, when it follows a noun. Ask whether it can be replaced with a defining clause with "that".
Look again at "Remember your son rebelling with a motorbike." Can it be recast as "Remember your son that rebelled with a motorbike." No. You are not remembering this son rather than another one! Son is already defined by "your".
To wrap up, we can ask "What if I was being asked to remember my son? Would I write the same sentence?" Actually, no. It would still be wrong. Without digressing into a technical discussion of why (but bear in mind that "the man looking like Elvis" is marginal in written English), I'll give the correct sentence and leave it at that. One would write: "Remember your son, rebelling with a motorbike." This construction is analogous with a "which" clause. You are being given more information about a previously defined noun. Which son? Your son. And here's more about him: he rebelled with a motorbike. You will nearly always want a comma when you use a participle after a noun that it describes. Look at "Here comes the king, walking slowly." Compare with "Look at the king, walking slowly" and "Look at the king's walking slowly." In the former, one is directed to look at the king, who is, as an aside, walking slowly. In the latter, one is directed to look at the act of walking slowly that the king is doing. The former is, of course, ambiguous. It can also convey that one is directed to look at the king while walking slowly oneself. You might prefer the first reading because it is more natural. But compare "Look at the king, taking your time". Now you very much prefer the reading that the participle defines you, although the other reading is again possible (as a matter of syntax; it would be hard to make a context in which it was semantically sound).
This leads us to a simple rule of thumb for deciding whether to possess "-ing" words. Ask whether you can write the sentence with a comma between the two nouns. If you are satisfied that you can, you do not have a gerund. If you feel you cannot, possess it.