Writing in Ye Olde Language
While it is certainly not required, many players like to roleplay by writing in language suitable to medieval times. This is a brief guide to how to do so in a manner consistent with how the language was (mostly) used at the time.
The Language Itself
When we write in archaic forms, we're not actually writing in Old English, or even Middle English. We're writing in a slightly outdated form of Modern English—which is, in fact, anachronistic, with how much depending on just when in the medieval period we're trying to emulate.
Old English was effectively another language altogether. You can see that in this excerpt from the tenth-century poem The Seafarer1:
Mæg ic be me sylfum soðgied wrecan, siþas secgan, hu ic geswincdagum earfoðhwile oft þrowade, bitre breostceare gebiden hæbbe, gecunnad in ceole cearselda fela, atol yþa gewealc, þær mec oft bigeat nearo nihtwaco æt nacan stefnan, þonne he be clifum cnossað.
In 1066, the Normans invaded England, and their conquest brought an old version of French, and a whole bunch of Latin, into the language. Over the next few centuries, the newly-blended language continued evolving; during this period it was called Middle English. It was now recognizable as English, but it still looked and sounded very different from how we know it today. Probably the most well-known example of Middle English today is the Canterbury Tales, by Geoffrey Chaucer. Here is a brief excerpt from the beginning of the Knight's Tale2:
Whilom, as olde stories tellen us,
Ther was a duc that highte Theseus;
Of Atthenes he was lord and governour,
And in his tyme swich a conquerour
That gretter was ther noon under the sonne.
So, as you can see, a few "thee"s and "thou"s don't make nearly enough difference for the language we use in BattleMaster to be anything but Modern English.
Thorn, Eth, Yogh, and Wynn
The "Ye" in "Ye Olde" is not actually a "Ye". It's a "The"—but as you can see above, English used to have more letters. Four in particular lasted some way into the medieval period:
Thorn (Þ) is the most common, and the most misconstrued. It was the sound of a soft "th", like you would find in "throne"—or, sometimes, the sound of a hard "th" as in "the". The letter itself bears some resemblance to a "Y"—and as movable type became common, "Y" was often used as a substitute for the thorn, which would not have always been available to the printer—which is why we retain the "Ye Olde" construction today.
Eth (ð) is related, but fell out of use earlier. It was more specifically used to denote the hard "th" sound, as in "father" or "this".
Yogh (ȝ) was used to make the "gh" sound—and, in fact, most of the words we have today with a "gh" used to have a yogh in that place.
Wynn (ƿ), which looks quite similar to the thorn, was eventually replaced by the letter w.
Ye, Thee and Thou
If you have studied other European languages, you have likely encountered the difference between singular/familiar and plural/formal forms of "you"—"tu" vs "vous" in French, "du" vs "Sie" in German, etc. The difference between "thou" and "you" is the same: "you" was the formal or plural form, while "thou" was the form you would use with one person you were on good terms with.
Similarly, if you have studied non-English Germanic or Romance languages, you will likely have come across the difference between subject and object forms of "you": "tu" vs "te"/"toi" in French, "du" vs "dich"/"dir" in German. "Thee" holds the same place in English with respect to "thou": "Thou art here" vs "I come to thee". "Ye" (yes, there's also a real word "ye") is the same with respect to you, but, somewhat confusingly, reversed: "Ye will all die" vs "I will kill all of you".
Doth, Dost, Maketh, Makest, and Art
There are also different verb forms that you can use when writing in archaic language. Again, this difference in conjugation is something that today's English has largely lost, but can be seen in other European languages.
In general, the rule is: If you're talking about what you are doing, or what more than one person is doing, you use the same form of the verb you would today. If you're talking about what the person you're talking to is doing (second person singular familiar—thou), you use the "dost" form. If you're talking about what one other person is doing (third person singular), you use the "doth" form.
So:
I do We do Thou dost Ye do He/she doth They do
Similarly, if the verb you're using adds an "s" in the third-person form in current Modern English—for instance "to make", which becomes "makes"—you generally add a "-st" in the second-person singular form ("thou makest") and a "-th" in the third-person singular form ("she maketh").
The one other verb that does something special is "to be", where the second-person singular becomes "art", as in "thou art".
Variations
This is, of course, all talking about "standard" English of the time. There have always been regionalisms, and dialects can take any of the rules above and bend or break them. The important thing is to remain consistent with yourself.
References
1. The Seafarer at Anglo-Saxons.net, http://www.anglo-saxons.net/hwaet/?do=get&type=text&id=Sfr
2. The Knight's Tale at Harvard's Chaucer Site, https://chaucer.fas.harvard.edu/pages/knights-tale-0