“Youse guys get off of the bed!” aka Accents and Slang.
Between two villains or two teenagers this line is perfectly acceptable. Spoken by a CEO, millionaire, or university graduate not so much unless it’s a deliberate attempt to be funny. An author doesn’t need to have perfect grammar. However an author does need to have a basic feel for how her characters speak. And characters in well paid positions, or characters who went to a good school, would generally speak grammatically unless they were deliberately playing a part.
Therefore when the narrative has many grammatical errors, this raises flags in the reader’s mind. Either the book needs good editing or the author needs to listen to her editor and make some corrections. There is no excuse for constantly mistaking their/there, or other simple errors, anymore than there is for having characters speak poorly.
On a similar note, if you are using dialects, be very careful to have them checked by a person who speaks that dialect fluently. Nothing is worse than having a creole speaker break into latin in the middle of a sentence. Babelfish and other such programs are not infallible, so always check what you’ve written.
Helen is available to line edit and/ or content edit fiction and non-fiction. Rates on application.