The one grammar rule that trips me up every time deals with commas and omitted second subjects.
Normally, if you’re creating a compound sentence, you would only add a comma before the coordinating conjunction (FANBOYS) if the second clause includes a second subject:
Correct: He forgot his keys, so he ran to the car. (He ran to his car)
That’s easy enough most of the time, but sometimes the subject is known and thus omitted from the second clause.
Incorrect: He forgot the keys, then ran back to the house.
Here, there shouldn’t be a comma, because I didn’t explicitly state a subject for the second clause. Old-school rules say I’m cutting off the clause from its subject, “he.” Unfortunately, I tend to think of anything prefixed by a coordinating conjunction as an independent clause, and add the second comma. Case in point? This sentence:
Unfortunately, I tend to think of anything prefixed by a coordinating conjunction as an independent clause, and add the second comma. (There’s no “I” in the second sentence, so I should have omitted the comma)
Alternatively, consider the second sentence:
There’s no “I” in the second sentence, so I should have omitted the comma. (Here, the second coordinating clause has its own subject, so I should keep the comma.)
Should the first example be called a complex sentence instead of a compound one? I don’t think so--that implies one clause has more weight than the other, when they’re actually pretty similar.
Here’s a proper complex sentence: one subordinate clause, one independent clause:
Although I love fruit, I refrained from eating at the fairy’s buffet.
It’s clear that the second half of the sentence matters more than the first half. I could cut off the first clause and the point would still stand. That’s generally the case with SUBORDINATing conjunctions--by definition, they make one clause subordinate to the other.
I tend to add the technically unnecessary comma to my longer sentences because the reader is unlikely to forget the subject of the sentence but can get lost in a long idea. Additionally, we tend to pause at the coordinating conjunctions, and commas often indicate pauses.
This is usually less of a problem in my academic writing, but I hate how often it comes up in my fiction. Sometimes I fight the system and sometimes I cave by deleting the comma or adding a subject to the second sentence.
(I know most of the rules instinctively but tutored a boy in English grammar and learned all of the names and rules by heart).
