For many of us, if we were ever required to do it at all, it has been ages since we've diagrammed a sentence. I actually have a book on how to do it on my wish list on Amazon. Even if we don't know the particulars of diagramming, most of us remember the bare-bones basics: Subject, Verb, Object. Stripped of their verbosity, most sentences contain these elements.
I want to share with you something that I see frequently in my work as an editor and show you the short-cut way of figuring it out.
One of the most common sentence structure errors I find involves participle and gerund phrases. Let's start with what those two dubers are:
. --petting her cat
This sentence structure is a tool for authors who want to show their character doing more than one thing at a time. You don't want to overuse it because "-ing" words are climbing high on the "don't do that" list. But the "-ing" form is not the only construction of the participle phrase:
.--doused by the water balloon
Both phrases before and after the comma pertain to the subject--which makes this structure different from a gerund phrase, which is the subject:
.--petting her cat
Can you see the difference? In the sentences above, the participle phrase modifies the subject; in this last one, the gerund phrase is the subject.
I haven't noticed a lot of problem with the gerund phrase, but the participle phrase tends to confuse people. Grammarians will tell you not to misplace or dangle your modifier, which is great if you know what a modifier is and how you lost it or left it dangling in the first place.
So here's the trick: Figure out the subject of the sentence.
Can you see what's wrong with this sentence? The implied subject of the first part of this sentence is Patricia; the expressed subject of the second part of the sentence is "it"--making the expressed subject of the entire sentence "it" (referring back to the cat). So, how does a cat pet itself?
A sentence with this construction should have only one subject. The subject of this sentence should be Patricia. So here's the quick fix:
petting the cat Patricia purring cat
Try this one:
This one's trickier, because it's intended to be in a deep POV. The implied subject of the first part is the unnamed character. The expressed subject of the sentence is the house--and if the house itself can walk from room to room, you have entered the Twilight Zone.
The oversimplified version of what I've been trying to say is this: Who/What is the expressed subject of the sentence, and is it the same as the implied subject of the first part of the sentence (the participle phrase). If they don't match, fix it.
Caveat: These are not the only sentence structures you can find a participle or gerund phrase in. These are just the structures I find most often when editing.
Give It a Comma
We're still talking subjects here, so bear with me. This one is a little trick to help you remember whether you need a comma with "and."
What's the subject here?
If you said "Lydia," you get to advance to the next level.
So, what's the subject here?
If you said, "That's a compound sentence, and you're trying to trick me," you've hit the nail on the head. So, can you tell me why the second sentence has a comma and the first one doesn't?
The subject of the first sentence example is Lydia. She scrubbed and wished--she's the subject of both sentence parts surrounding the "and."
The subjects of the second sentence example are Lydia and Mark. The subject of the first part is Lydia, the subject of the second part is Mark. You stick the comma before "and" because you've got two subjects thrown together in one sentence. Or, you can say you have two sentences thrown together with the conjunction, "and."
Let's play with this some:
All of these are right. The subject of the sentences is Lydia, but you've got her doing a list of things: scrubbing, rinsing, and wishing.
In the first sentence, the activities are separated by "and," and no comma is necessary since the same subject is doing all the action.
The second sentence uses a comma to omit the first "and," emphasizing the list of activities. According to the Chicago Manual of Style (the book publisher's Bible), lists of three or more things (activities in this case) are divided by a serial comma--including a comma before "and." If you're ever in doubt, strip the sentence down to its subject and verbs. If it turns out to be a list of activities, use the comma, even before the "and."
The third sentence is a subjective use of the comma. I'm sure there's some technical term for it, but use of the comma here is more artistic than standardized. Since the first two activities are related, they aren't separated by a comma. But the third, the "wish," expresses a mood, a desire, an internal emotion that the author may want to emphasize as separate from the rest. Putting the comma there separates that phrase from the practical to the wistful, illustrating a difference in tone for that part of the sentence.
Another way to write that last sentence is with a participle phrase (going full circle here):
wishing Mark was with herWishing Mark was with her
The construction of these last three examples, which emphasize the "wishing Mark was with her" part, is entirely up to the author. Personally, I like the one without the participle phrase best--and if you use the participle phrase too often, you may have to change it.
There's no comma in this sentence because both parts are short. According to CMOS, that's fine--but also according to CMOS, if you want to put a comma there, you can. Doncha love it?
Recognizing your subject helps in your sentence construction and comma placement. It isn't "Grammar 101," which indicates a college level course. It's "Grammar School." I'm sure you remember all this; you just needed a little nudge to the gray matter. Glad to help (assuming I did).