there are three lamb faces in this photo-- find the third!
In conversations, people don't talk with grammar in mind; so dialogue is a time out, where I ignore Word warnings, despite it going tsk tsk. Dialogue has each character with different education levels and way of talking. Cannot use a 'big' word for someone who wouldn't know it-- unless there is a special reason for their knowing it.
Prose is where grammar is more of a factor-- transitional, descriptive, and point of view sentences. In point of view, the character is thinking or observing. I don't feel it's needful that it sound like they talk, but it shouldn't have words that don't fit what that character would observe and know. If a character is the type to enjoy a sunset, the descriptive words should fit their nature. Poets sound like poets. Most other people do not. Fixing all of that, making it sound and feel right, is what editing is about.
My first draft will always be written fast without a lot of concern for the nuances of grammar. Because I grew up when diagramming sentences was part of learning to write, I generally put my sentences together with most of them correct. There are always some where my mind is ahead of my fingers, and the words prove it.
When editing comes along, I read for meaning and feeling and use Word underlining. I like the option of having it check what I most need. Sometimes, even in prose, I ignore the underline. More often I change it-- if not literally as they would suggest, but with a rewrite that avoids the underline. Sometimes it'll start with part of a sentence underlined and the suggestion it needs a comma. As soon as I add the comma, the whole sentence is underlined with a new complaint. Luckily Word isn't a person where we could get in an argument over it.
One of their interesting grammar warnings is-- you're too wordy. Most writers can definitely be that way. Seeing that warning is when I decide if the whole sentence is needed. If it is, I usually break the thought into several sentences. That can lead to a new warning.
The main problem I have with them is when something sounds right but is grammatically incorrect. Dangling prepositions are one example. When we talk, we don't worry about them. In dialogue I don't either. But when I'm writing a transitional or descriptive paragraph, they generally need to go-- not because they bother me, but because they might some readers.
There are some places Word isn't good at catching, and I really have to watch myself. An example is worse and worst. Theoretically, it's simple. Worse is in a chain of bad. Worst is the absolute most it can be... except sometimes either one just does not sound right even when it probably is. When that happens, I grab my thesaurus for an option to avoid fighting with myself over it or annoying a reader where that very word is their pet peeve.
Another place English can get confusing is for the verb was, which must fit its noun. Was is generally used with a singular noun, which means you don't write 'we was going to town' that is unless your character is uneducated and from the hills. But was isn't always correct for a singular noun. It changes when it's in certain clauses, which means were is correct for a singular noun-- even when to me it sounds
The first times I came across this correction to was, I argued with the program and assumed it was wrong. It was not. In fiction writing, I still though won't use what sounds awkward just to make my grammar pass an English teacher's red check.
Another one, that Word regularly catches, but in a rough draft I often write incorrectly, is the split infinitive. I don't really know why it's so bad (it always sounds fine to me), but I change it when I see the warning. It might bother a reader. As you may have already deduced, editing is really about the reader. I already know what I intended to say.
Word is a great program for helping the editor. Some buy more complicated programs, but I've tried a few. They aren't always better. In fiction, correct isn't always best. If it comes across as awkward, it's not best even if it's most grammatically correct. Who and whom come to mind for that. I get it when whom is the right word to use, but not if it is too highfaluting for a character whose point of view is simple.
I can't emphasize enough how important it is to get point of view right for the character. A simple woman, in her thought life, won't use words she would not use talking to a friend. Writers generally have quite wide vocabularies, but where it comes to point of view, they can't use them all no matter how tempting.
This has recently been my world as I went over my Oregon historical to make sure it passed muster-- which is very important to me. I want my book to go out with zero errors. I am not sure they ever do.
Editing is about a lot more than Word's warnings, but they force me to look at a passage for a clearer way to make my point. I recognize though that sometimes a round about way of saying something is better, when the goal is create a word picture for the reader.
If you didn't get enough grammar yet, give this article from the New Yorker a try--