Thart in the right place. Please, proceed and click.
The English language has changed a great deal over the course of its history. It's been supplemented from its original wordstock by Norse, French, Latin, Greek and countless other languages. The influence of these languages, and of other cultures, has caused English to morph substantially.
But say you're a writer. Or an historical re-enactor. Or a memesmith. Wouldn't you like your work to not only evoke the time period you're replicating, but actually work?
To that end I'm beginning a new series of posts aimed at giving a few tips toward improving the verisimilitude of your attempts to capture the speech of bygone days.
Thou must be kidding me
The word 'thou' is the first lesson. But what do you do with 'thou'? That depends on the period you're trying to evoke. So, I'm going to give you a quick primer on thou through the ages, followed by how to actually use the word. There are some dialects that still make use of forms of 'thou'; these examples are to illustrate the place of the word in the standard vernacular.
Thou through the Ages
20th century: 'Thou' is dead as a doornail here, used rarely (and often incorrectly) to provide flavor and not for any reasonable purpose.
19th century: 'Thou' is a ghost here. It serves two purposes, really: making something sound old-timey, and being a form of direct address in poetry.
18th century: My friend Joseph Ducreux is using 'thou' up there in that meme, but he's using it wrong. No surprise, given he's an 18th century Frenchman and not English. Also no surprise that 'thou' has about the same range of use in the 18th century as in the 19th century.
17th century: 'Thou' is still in use here, but it's rapidly falling into its 18th century usage patterns. When used in earnest, 'thou' is used when speaking to someone of a lower social position than you occupy. Otherwise it's largely poetic.
There's also Quaker plain speech, which emerges around this time. In short: 'thee' can be both the subject and object form, and you often use the 'he/she/it' form of the verb. 'Thee is interesting' is correct only in Quaker plain speech. Plain speech derives from a conscious effort at social equality by eliminating the T-V distinction (more on that later, but standard English eliminated the distinction by eliminating 'thou') through giving everyone the intimate form of address. For 18th and 19th century Quakers translate "You have" to "Thee hath." In the 20th century this becomes "Thee have."
14th through 16th centuries (inclusive of the 'Late Middle Ages'): This is truer beginning toward the later 14th century, but the end of the Middle Ages and the beginning of the English Renaissance feature the strongest French cultural influence on the use of the English language. This is the golden age of the T-V distinction in English, where you differentiate your pronoun choice based on the relative social standing of the person you're talking to. So, as in the 17th century, 'thou' is for someone of a lower social position.
pre-14th century: 'Thou' is the singular second person pronoun. Social standing is not important.
But what dostow mean?
So, thus far you've got some idea of when to use 'thou' in a piece of writing (if you're going to). Now we're going to get to the how. The use of 'thou' breaks down into three simple facets. There's understanding the T-V distinction, understanding how to conjugate verbs for it, and understanding how to decline the pronoun.
Thou and You
The T-V distinction is named for the way the French use their pronouns tu (thou) and vous (you). 'Thou' is singular. It only gets used to refer to an individual - no matter what century you're attempting to imitate, 'thou' never refers to more than one person (as Mr. Ducreux does above). So, when referring to a group in English, you're always going to use a version of you when talking to multiple people.
It's when talking to an individual that the T-V distinction occurs. If you occupy a higher social standing, you would refer to the person with 'thou.' You would also use 'thou' if you want to be passive agressively dismissive of them when they should be respected. You only refer to an individual person with the pronoun 'you' ('ye', actually, but that's another lesson for another day) if you're showing them respect or deference.
That's what you do to determine who is a thou (if you have to, prior to the Late Middle Ages there's no need to do that, and after the Renaissance there really isn't either). But how about using it?
What canst thou do?
When using 'thou' you have to remember one thing about its verbs. They change, just like third person singular verbs change. All of the verbs in English used to change. We've trimmed off most of those changes, but we still have residual familiarity with how 'thou' works - mostly thanks to the Renaissance. Shakespeare's our friend.
So when you use 'thou' as the subject of a verb, you're going to have to add an ending. That ending will, with the exception of a few weird verbs which I'll cover shortly, be -(e)st.
You can include the e or not, depending on how you'd like the word pronounced. Some verbs never really take the e: do becomes dost, can becomes canst, that sort of thing. A good resource would be the Middle English Dictionary. From this link you can search for a verb. You'll have to deal with Middle English spellings for the headwords, but if the word existed back then you can get the most accurate feel for it this way. When you find the headword for the verb you want (say you're looking for the verb 'to gather' - that would be under 'gaderen (v.)') you should then click to show quotations and start searching for 'thou' (or other variations, including thu and thow).
Once you have your verb ending, you jam it on to the end of your verb. Wham, bam, thank thee ma'am. Well, mostly. If your verb ends in er, like our friend 'gather', or in any combination of -e(X) as its final syllable you'll want to absorb the vowel. So, in the case of 'gather' you drop the e from the base verb and add the -est ending in full, getting 'gathrest' as a result. If a verb already ends in an e and there's another verb that would be identical without the e (singe and sing, for instance), I recommend adding only -st to both. This lets you preserve the differentiation between the words more easily (singe becomes sin-gest, while sing becomes singst).
Present tense is pretty simple. Past tense is too, mostly. You do basically the same thing as in the present tense, except you make the verb past tense first. So 'do' becomes 'didst', can becomes 'couldst', singe becomes 'singedest.' The tricky part is the strong verbs, the ones that don't make their past tense from an -ed suffix but by changing vowels (like sing). If you're going for anything post-twelfth century, you're fine putting the verb in its modern past tense and adding the suffix. Sing becomes sangest.
If you're going for an Early Middle Ages feel (pre-twelfth century), however, you have an extra step. It's not exactly the rule governing how it worked in Old English, but what you want to do is take the past participle of the verb (the one you use after a form of have). So, sing - sang - sung (or drive - drove - driven, to give the two types of examples we need). If there's an -en, remove it (you can keep the e if it won't affect pronunciation); if not, leave the participle alone. That's your thou conjugation in that past tense for the strong verbs. Thou singst today. Thou sung yesterday. Thou drivest today. Thou driv yesterday.
There are a few kind of weird ones. 'To be' becomes 'art' and 'wast' ('wert' in the subjunctive). 'Will' just becomes 'wilt'. Must stays the same.
To do a command, all you need to do is the same thing you would do now: drop the thou ending from the verb and use the bare infinitive. 'Thou singest' means that thou art singing. 'Thou sing' means that I am telling thee to sing, and I mean do it now.
Lastly, there are some fun contractions to add spice to your use of 'thou'. Among the easiest to include are the ones built for questions. 'Artow' (art thou), 'hastow' (hast thou), wiltow (wilt thou), knowstow (knowest thou) are pretty simple to include. Also worth knowing is that there is a 'you're' equivalent: 'thart' (thou art).
Just like with any other pronoun, you only need to conjugate the verb for the one the pronoun is doing. He eats - he will eat. Thou eatest - thou wilt eat. You never conjugate the verb after a modal verb. 'Thou must keep it real', not 'thou must keepest it real'.
One of the great things about having a conjugation that exists only for a particular grammatical person is being able to completely leave out the pronoun and still maintain clarity. 'Why carest?' gives you everything you need to know about the subject of that verb in the verb itself. It's really useful to be able to do that.
And that's verbs.
Thee, Thyself, and Thourene
There's only one thing left to do, and that's learn how the other forms of the word 'thou' come into the picture. Fortunately, 'thou' is no different from other personal pronouns and it's not really that hard.
First thing's first. 'Thou' is the nominative form of the pronoun. That means that when you have a verb, and you want a 'thou' to do it, the 'thou' is the subject. Thou canst never use 'thee' or 'thy' or 'thine' as the subject of a verb.
Next there's 'thee': 'thee' is the dative/accusative form of the pronoun, but you might grok it better if I call it the objective form. 'Thee' is analogous to me. If you want to be sure you're using 'thee' right, change the person to first or third masculine (feminine introduces the ambiguity of her, which functions as possessive as well, which is why I recommend the masculine). If you would use 'him' or 'me', that's when you use 'thee.'
Lastly, there are 'thy' and 'thine'. They're the genitive (possessive) forms of the pronoun. Here's how they work:
If you're going for a pre-1200 English feel, you'll want to just use 'thine.' Any time you would use 'his' with a third person or 'my/mine' with a first person, use 'thine' and you'll be golden.
If you're going for a post-1200 English feel, things get more complex. You use 'thy' any time the next word/phrase (usually the noun phrase indicating the object possessed) begins with a consonant sound: Merlin is thy name. You use 'thine' any time the noun phrase begins with a vowel sound: Keep thine honor. You also use 'thine' in any case where you would use 'mine' in Modern English: Should I say that the pleasure is all thine?
There's also 'thyself', the reflexive pronoun you use when the 'thou' performs the action on itself: Thou canst not control thyself.
That's more or less all there is to it. Now thou canst go forth and write.