Speaking a language is a skill that we continue to do all our lives. If a child hears the language spoken properly, they will learn to speak it correctly without thinking about it. At least they will be speaking it correctly according to those that decide what is proper speech.
How we speak has been determined by history, especially the history of empires. When the Anglo-Saxons came to the British Isles their brand of English was considered the right language and the Celtic language of the Britons 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. Theirs are much more consistent 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. But, language is always changing. People now may think that ordinary ways of speaking 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.” One character frequently used filler language such as “actually” or “on the other hand” to create emphasis. An editor marked these out as unnecessary phrases, but I protested. If all people in fiction spoke the same way as prescribed by some famous writer, dialog would be boring.
Unfortunately, we pay too much attention to the advice of a few best-selling authors. 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 centuries ago from Anglo-Saxon to Old French which was reversed later, the preferred style of writing may swing back to what it used to be.