Something that is especially common in fantasy stories is the use of archaic pronouns and verb forms. This is usually accompanied by other archaic constructions. Commonly this is referred to as “old English” or “ye olde english” (pronunced “yee oldee english” which, incidentally, is incorrect). The errors are most commonly made by amateur writers but I have seen it from professionals whose work should have passed the desk of at least one competent editor.I should clarify that I am specifically referring to the old first person singular pronoun “thou” and the verb conjugations that go with it. I am also referring to the contemporaneous third person singular conjugation. You will be most familiar with these as “thou art”, “thou hast”, “he hath”, “thou speakest”, “he speaketh”, and so on. There is nothing particularly special about the third person form. It has simply been replaced by the more common forms we use today (“he has”, “he speaks”).
“Thou” and its other forms (“thee”, “thy”, and “thine”), on the other hand, served a specific purpose: they represent the second person singular. Originally, there was no special connotation to this form. Eventually, like in other languages, it took on a sense of familiarity (similar to “tu” in French) while “you” came to be used in formal circumstances and when indicating respect. (“You” was the objective form of “ye”, the plural equivalent of “thou”.) The circumstances were muddied a bit but the use of “thou” and “thee” in biblical translations. This led to “thou” and its other forms achieving an air of respect at odds with the older connotation of familiarity. This particular confusion may have contributed partly to its decline and eventual disappearance from general usage.
It is worth noting that if you are going to use “thou” and “thee” and the like, you do not need to adhere strictly to the cultural conventions of our past if your context is not historical. However, you do need to be consistent, and you do need to use the specific forms correctly. You cannot simply append “-est” to every verb (or worse, every word) and gain an air of mystery or medieval culture. Do not write such drivel as “Fred livest hereth” or whatever. (“Fred liveth here” would pass, however.)
While the use of “thee” and “thou”, “ye” and related forms is the most visible thing that is often done poorly, it is certainly not the only thing. There are other words that also conjure up a similar air. These are words like “hither”, “thither”, “whither”, “hence”, “thence”, “whence”, “wherefore”, “anon”, and the like. Make sure you know what the words mean before you start throwing them around. This is especially important for words like “wherefore” which do not mean anything like what they appear to.
Finally, simply looking at some old text or phrases does not give you any idea what you are looking at. We have all seen things like “Ye Olde Shoppe”. That is most definitely not pronounced correctly as “Yee Oldee Shoppee”. The extra “e” at the end of “olde” and “shoppe” is silent and is an artifact of old spelling forms. The “ye” is not actually the pronoun “ye” mentioned previously, but is, in fact, the old, familiar word “the”. That’s right. “Ye Olde Shoppe” is actually no more fancy, when spoken, than “The Old Shop”. On the other hand, the old familiar phrase “hear ye!” actually is the prounoun “ye”. It’s an inverted form of “You hear!” meaing “Listen up, everyone!”. So where did the “Ye” thing in “Ye Olde Shoppe” come from? Well, way back, English used to use more letters and “The” was often written with a letter thorn (Þ uppercase, þ lowercase). When printing presses came along, they didn’t have fonts with thorn so they picked a letter which looked similar (for some value of “similar”) and that turned out to be “Y”. (You’ll have to imagine handwriting at the time and consider that the letter forms have changed a fair bit since the printing press first arrived.) This occurred before the “th” construction gained popularity as a replacement. We can attribute the disappearance of a few other letters to the advent of the printing press, too.
By now, if you’ve bothered to read (or skim) this far, you’re probably wondering what the point of all this is. The point is simply this: do your research before you attempt to use any archaic language forms. You almost certainly do not understand their usage and, as the discussion of “ye” shows, you cannot rely on your intuition to figure it out. Understand the context for the language you are borrowing and only then attempt to use it in your new context. Remember, using archaic language in a manner that is too far from how it was actually used, or too far from correct, will almost certainly annoy that fraction of your audience who know the correct usage, and it will possibly confuse others. Assume your audience is literate and familiar with the forms and words you use. That way, you don’t annoy those who are familiar and you don’t perpetuate bad usage among those who are not familiar.