A huge flaw in many novels set in earlier times is that the author has failed to use language appropriate to that period. If you are a fiction writer, this is something you must pay attention to. You spend a great deal of time accurately describing the setting, the clothing, and the buildings. You need to be just as accurate about the language your characters use. If you have set your story in earlier days, your characters can’t use modern day English—they must use words and phrases which were common at the time of your story.
Many years ago, I was attending my first Western Writers of America conference (WWA–a great organization, by the way). On the bus coming into town from the airport, I met a number of the other attendees. One of these attendees was the writer, Chet Cunningham, author of hundreds of western and military novels as well as a number of non-fiction books. In one of our conversations in the days that followed, Chet told me about his lists of language appropriate to the periods where he placed his stories. He was very particular about when words were introduced into the English language and then again when they faded into obscurity.
Since I don’t write historical fiction, I had never really thought about this issue before Chet brought it up. At that point always watching for this issue, I began to watch movies listening to the spoken words; I read books with a critical eye on the dialogue. In my mind, the credibility of the screenwriter or author went up or down in regard to their language appropriate accuracy. True Grit, the movie, is an example of writing historically accurate dialogue. Although contractions had been around for centuries, at that period of time (1880s), correct spoken English did not use contractions. The producers and original film adapters, Joel and Ethan Coen, stayed true to the era and kept the film English very formal. (Marguerite Roberts wrote her screenplay true to the Coen version.) This unique dialogue was an important factor in making True Grit an award-winning movie. Accuracy matters, even when you are writing fiction.
Before you start to write a period piece, spend some time reading books and articles which were contemporary to your time frame. You will begin to hear the cadence and vocabulary of the language used at that time. Further, if you are unsure as to whether a word or phrase was used at that time, simply google it. The internet is a fount of information, so you can also look up etymology and follow the sources there. Work on your dialogue so that it is true to the setting of your story.
Some Sources to Find Language Appropriate
http://www.etymonline.com/ This is an online etymology dictionary that covers most eras of English language.
http://www.oxfordreference.com/view/10.1093/acref/9780199547920.001.0001/acref-9780199547920 Oxford Dictionary of Word Origins. This is internet version of the original go-to hardcover of etymology.
http://www.wordorigins.org/ An interesting assortment of short blogs on language.
http://mentalfloss.com/article/53529/56-delightful-victorian-slang-terms-you-should-be-using fun slang from the Victorian era
http://www.elizabethan-era.org.uk/elizabethan-language.htm great info smothered in popup ads. If you can stand all of that and get to the core, you can find some great Shakespearean insults, etc. Although the info was good, I really was annoyed by all of the ads.
https://quizlet.com/4851287/cold-war-era-vocabulary-flash-cards/ list of terms used during the cold war era.
https://quizlet.com search for the era you are interested in and then see the lists of vocabulary words posted for that era. This is rather hit-and-miss, but you might find a nugget here.
http://celticfringe.net/history/vocab.htm words from the 1830s
http://ask.metafilter.com/4730/Language-in-the-18th-and-19th-century Q&A about how to find the vocabulary from different eras.
http://www.todayifoundout.com/index.php/2014/11/origins-english-contractions/ interesting blog on English contractions and how/when they were used.
http://www.history.org/history/teaching/enewsletter/june03/english.cfm 18th century English