A couple of comments I’ve had (and welcome) on my shared work (Therein Lies the Rub) and which would also be equally relevant to many of my short stories relates to how I depict speech.
Whilst I understand the need to reflect the different language that people would use in terms of their background, education, mood etc., it has been suggested that I depict speech to reflect how it is actually spoken.
e.g. “Yer gonna regret that,” rather than the way I would currently write: “You’re going to regret that,” and leave the accent to the reader’s imagination.
I sometimes find it quite tiresome to read speech written in the former manner and almost (dare I say it?) slightly patronising which is why I’m tempted to refrain from using it – but is this at the expense of realism?
I'm going to go against everyone else here lol
I'd find it more grating for a character to be from, for example, Alabama, and suddenly start speaking the Queen's english. You know?
I was told that you can write whatever you want as speech - poor sentence structure and whatever. Just as long as the non-dialogue is correct (obviously).
In my own work, none of my characters have thick accent's - and that's only where I'd use it: characters with thick accents - but my protagonist is an 18 yr old girl from Cambridge. And since I was that girl a few years ago, I use the appropriate language for her age.
I think your headline question may be slightly misleading. "Correct English" throughout dialogue? Probably not. How many people do you know use "whom" correctly in speech? Not many - and the few that do will be well characterised if you make a point of it. Punctuation, I think, can also be relaxed: for example, what might look like an ugly run-on sentence in narrative can look lively and realistic in dialogue.
But to go as far as your example of "Yer gonna" ... speaking personally, please don't. I for one find it very tiresome, especially if, having started, an author feels obliged to keep it up for 300 pages. Be subtle - just a few cues should be enough to convey the differences between characters.
This is an issue I had, but never realised until one of my proof readers mentioned it. I wrote all my dialogue without any contractions, we are instead of we're for example.
Believe it or not but I actually talk like that most of the time, and even when text messaging, when most grammar is thrown away, I write as I talk.
It was explained to me as a matter of flow, and believability with the language used.
If you think your character would talk like that then I think they must, your are the creator here and only you know what your characters do and think. But I would also be careful about over using slang in your dialogue