The biggest thing I notice about a lot of writing is not whether the grammar is correct, but whether the dialogue sounds natural and native. So it is related to grammar, but not everyone speaks correctly. So when I hear "If you would have done this", it grates, but it is believable that someone would say it. I can even just about, through gritted teeth, allow "could/should/would of" in *dialogue*. Not in the narrative, except maybe, just maybe, in first person POV. Just don't do it if your character is a pirate, yarr.
