How we speak has been determined by history, especially the history of empires. When the Anglo-Saxons came to the British Isles their germanic brand of “English” overwhelmed the Celtic language of the Britons, and the older tongue was considered the language of the less educated. When the Norman Conquest occurred, old French became the correct way to speak and write, while the Anglo-Saxon version of English, which had changed quite a bit since they arrived, became a lower class language. However, when the Norman nobility fell out of favor people stopped speaking old French even though a lot of this vocabulary remained in the English language, and we even use it today.
One of the artifacts of these changes is the number of irregular conjugations in English. For example, the past tense of verbs are formed in numerous different ways without much logic. “I see” becomes “I saw” and “I sing” becomes “I sang.” Some people have failed to learn standard verb tenses because they heard English spoken incorrectly and therefore, they say, “I seen” and “I been.” You notice those last two verbs should rhyme but don’t. French people who learn English struggle with our inconsistent pronunciations that much more consistent pronunciations based on how words are spelled. But, the French did not get a huge influx of English vocabulary into their language, at least until recently.
How we learn to speak cannot be separated from the way that we write. Authors often compose their sentences similar to the manner that people speak in their day. But, language is always changing. People now may think that proper language from as little as fifty years ago is pretentious. The difference is even greater for language used more than a century ago. Each author needs to develop their own style, while still using enough current language for their writing to be intelligible to readers.
When I write dialog, my characters do not speak the same. Some don’t use proper English and say “I seen.” Another character used a lot of filler language such as “actually” or “on the other hand” frequently to create emphasis. One of my beta readers marked it out as unnecessary phrases. If all people in fiction spoke the same way, dialog would be boring.
Someone from one hundred years ago might consider what I think is correct English as poor writing.
I find some current writers using phrases that are not sentences, or including slang understood only by a small percentage of the population. Someone from one hundred years ago would consider that poor writing. So, we should not judge the writing style of past authors so harshly. Like the switch in Britain from Celtic to Anglo-Saxon to Old French which was reversed later. There were so many variations in the English language a “received pronunciation” originating from royalty was required to establish standards for speaking English. The preferred style of writing may always swing back to what it used to be.
Photo by K.N. Listman
Write about what? 
So true. I’m ever aware of the differences of speech when I have adult and child characters speaking. Whereas the former would likely say, “I dived into the pool,” the latter would say, “I dove in the pool.” I had a beta reader try to correct the latter example when my young character claimed to have dove headfirst instead of dived. Another example had a character warsh instead of wash. The dialogue (not dialog) represented their upbringing in the Appalachian area. It confused my beta readers (and spellchecker on my computer) when my character pronounced water as wooter and creek as crick. Using y’all and yonder were easier to pull off. It’s all great fun for me when my characters speak.
I try to limit words spelled as pronounced rather than spelled correctly to less than 10% of the dialog so it doesn’t get too difficult to read. However, including only 5% is enough to get the point across.