I was watching the 12 Monkeys series and at one point a character says, “Alloweth him to see!”
This was funny to me. This is exactly the same as saying, “Allows him to see!” in modern English. You can’t use this inflection for imperatives. Most English speakers won’t notice this kind of mistake, but a few will. Those who’ve spent a significant time reading the King James Bible, for example, will probably pick up an intuitive sense of Early Modern English (EME) grammar, even if they can’t explain the rules themselves.
Here are some pointers for fiction writers. They will help you avoid some of the more obvious mistakes.
The th verb ending is the same as the modern English s verb ending. For example, saith → says. Like the modern s inflection, it is only used for third person singular subjects. (I.e., the pronouns he, she, and it, and any words they could conceivably represent.)
Th doesn’t get added to anything that’s not a verb. Unless you’re specifically making fun of poor understanding of EME, don’t say something like everydayeth. If you are making such a joke, make it very obvious.
Steward Ribson in Brave Fencer Musashi is a great example of such mockery. “Thou art beeth correcteth! However, they willeth not sitteth around quietly. They willeth come hither for Lumina!”
The t/st/est verb endings are second person singular, for both singular and past tense. E.g., present: thou art, thou canst, thou standest. Past: thou wert, thou couldst, thou stoodest. If you’re not sure how to inflect, see if the verb is in the King James Bible, which is easily searchable online. (Shakespeare is actually a bit between EME and modern English, and switches between rulesets—sometimes within the same sentence—so he’s not as reliable a resource as the KJV Bible. But if the Bible’s a bust, you can try Shakespeare as well.)
Thou is nominative/subjective second person singular. Use it where you would use I/he/she. Thee is oblique/objective. Use it where you would use me/him/her. You and ye are plural, but are also used for formal singular. Your characters shouldn’t call the king thou (unless they’re trying to be rude or are clueless).
The y in the other Ye, as in Ye Olde Shoppe, is supposed to be a typeset substitute for the letter Þ, called “thorn”, from Old English. Ye in this case is the word the and is pronounced the same way. EME characters shouldn’t say this ye with the “y” sound.
The rules for choosing between my or mine and thy or thine are the same as the rules for choosing between a or an in modern English. My and thy precede words that begin with a consonant sound. My shield, thy sword. Mine and thine precede words that begin with a vowel sound. Mine axe, thine armor.
H was typically not pronounced at the start of words, much as the modern word hour, so people would use mine/thine preceding them. E.g. mine heart sounds like mine art, and my heart would be ungrammatical.
If you get the eth verb ending and the difference between thy and thine right, you’re already ahead of the curve.
If you only want to consume rather than create, knowing these rules can… mess up some fiction for you. Sorry. But it can also enhance it, as some writers do take care to get these things right, and it’s cool when they do.
In the end, the characters still need to be comprehensible to modern audiences. Prioritize comprehensibility over keeping to these rules. For example, the rule of not pronouncing h at the start of a word should probably be discarded in written fiction. Readers will think of the modern pronunciation, and an horse will look weird.