So, you've decided to write a piece of historical fiction. You've researched the period for key events and clothing, but one thing it's important not to forget it language. It's easy to recall that an eighteenth century character won't have a smart phone; it's not always so easy to remember which words, common now, were not coined until after 1800.
As a reader, I hate reading an historical story in which the language is incorrect for the period; therefore, as a writer, I try to ensure my works keep as true as possible to the era in which they are set. Naturally, there are limits. When I wrote a story set in the eighth century, I obviously could not pen the whole thing in Old English. However, when writing, say, an eighteenth century setting, it is possible to be authentic while remaining intelligible to a modern audience. So here are a few pieces of advice.
1) Check for First Use
If you are in anyway unsure about a choice of word, check it's first recorded use. I do so by searching for it on Merriam Webster's online dictionary, which lists this information for each entry.
2) Work in Relevant Slang/Contemporary Vocabulary
This is easier for some periods than others, but generally from 1600 onwards you can find slang lists online or in books. Georgian/Regency cant and Victorian crime slang offer particularly rich pickings. If this is not an option, you can always show period in the dialogue.
3) Pay Special Attention to Dialogue
Where possible, I stick to era-appropriate prose and dialogue, but in cases where this is problematic, you can at least keep your dialogue sounding correct. This is easier from 1600 onwards, as already noted, but if, say, you are writing a piece set in Ancient Rome, you could have them speak English but with the occasional Latin word thrown in (perhaps for swearing or ejaculations) to give a sense of time and place.
4) Check Contemporary Works
You can always refer to works written in the period (when available) to get a feel for both style and vocabulary, especially with regards to dialogue.