Let’s see how tight I can keep this. Every sentence has a subject and a verb. Dogs bark. Cats purr. Most sentences also have an object. Most cats have fur. Sentences have spines that way. Almost everything else in a sentence is some kind of modifier, which can be a single word. Collies have long fur. Or a phrase, of the sort that starts with a preposition. Dogs of all kinds chase cats. Or a clause. Dogs, which I have always owned, need exercise. When all the modifiers are attached to the correct subject, verb, object, a sentence is grammatical. When not all of them are, the author has broken grammar, and a copy editor fixes the sentence.
With any copy edit, checking grammar is the bedrock task. It’s called parsing, and it’s what you do with every sentence, one after the next, without fail or exception, for however many hundred pages are in front of you. Subject, verb, object, modifiers, check, subject, verb, object, modifiers, check. Everything else you do during the actual copy edit starts with that. Welcome to your day.
Grammar and syntax are not the same thing. In fact, the two have little to do with each other, but a CE is expected to be competent with both and attentive to both. With every sentence, parsing is step one, syntax is step two. A metaphor would be useful: grammar is under the hood, syntax is on the dashboard. Grammar is the rules language needs in order to work. Syntax is about how the rules are applied. Some people are far better drivers (use far better syntax) than others, but without a functioning engine (rules of grammar), their car is not going to reach its destination, no matter how good the driver.
Parsing is a black-and-white exercise. A sentence either is or is not grammatically correct. You can override an author any time to fix their grammar. Syntax is a very different beast, and adjusting it can be a fraught exercise. You have to triangulate between the house’s expectations, the author’s perception of his own skill, and your own hard-earned judgment.
Re the house’s expectations, I’ve known some that say flat out, “If the sentence is grammatically correct, don’t change it.” Strange that it’s the most prestigious houses that take that attitude – I’m referring to Ivy League or Ivy League West houses. It seems that some houses are more easily cowed than others. I’ll hear, “Go easy – this woman is an emerita at Hallowed Halls University,” and a mention of how many books she’s already published. I ignore that instruction, falling back on the mantra, “Everything I do is advice,” which has the benefit of being true. All of it is advice, and they’re going to listen to it, by God. Whether an author actually takes that advice, and how much, is going to be a function of how highly she thinks of her own writing (leaving justification aside) and how good you are at your job. That is, how good your own ear is, and how well attuned you are to the manuscript in front of you.
BTW, I’ve found that authors who have been published extensively are the easiest ones to deal with, because they’re used to being copy edited. They already know what they’re in for and are the ones most likely to see why you made the changes you did. An author who responds to your CE as if you just shot their dog is, most of the time, a first-timer.
More about syntax later on. I’m trying to keep these posts to one page.
ADDENDUM: Most first-time authors are so eager to get their book out, and so unfamiliar with the process, that they offer little pushback to the CE. In fact, CEs prefer to encounter some pushback from the author, because it indicates they looked hard at your work.