I write things...sometimes

They play the same roles as I/me/my/mine. In modern english, we use “you” for both the subject and the direct object/object of preposition/etc, so it’s difficult to compare “thou” to “you”.

So the trick is this: if you are trying to turn something Olde, first turn every “you” into first-person and then replace it like so:

“I” →  “thou”

“Me” →  “thee”

“My” →  “thy”

“Mine” →  “thine”

Let’s suppose we had the sentences “You have a cow. He gave it to you. It is your cow. The cow is yours”.

We could first imagine it in the first person-

“I have a cow. He gave it to me. It is my cow. The cow is mine”.

And then replace it-

I sleep in the attic. Thou sleepest in the attic. He sleeps in the attic.

I love pickles. Thou lovest pickles. He loves pickles.

I go to school. Thou goest to school. He goes to school.

The -(e)st ending is only added to one word in a compound verb. This is where a lot of people make mistakes:

I will believe it when I see it. Thou wilt believe it when thou seest it. He will believe it when he sees it.

NOT

*thou willst believest it! NOPE! This is wrong

If you’re not sure, try saying it in the third person and replacing the -(e)st with -s:

*He will believes it when he sees it. ALSO NOPE! 

In general, if there’s one auxiliary, it takes the -(e)st ending) and the main verb does not. If there are multiple auxiliaries, only one of them takes -(e)st:

I could eat a horse. Thou couldst eat a horse. He could eat a horse.

I should go. Thou shouldst go. He should go.

I would have gone. Thou wouldst have gone. He wouldst have gone. 

You can reduce the full -est ending to -st in poetry, if you need to drop a syllable:

thou sleepst, thou lov'st.

In some common words–mostly auxiliary verbs, or what you might have learned as “helping verbs”–the ending is always reduced:

I can swim. Thou canst swim. He can swim.

Sometimes this reduction takes the last consonant of the stem with it:

I have a cow. Thou hast a cow. He has a cow. 

Or reduces the -st down to -t:

I must believe her. Thou must believe her. He must believe her.

I shall not kill. Thou shalt not kill. He shall not kill.

However! UNLIKE the third-person singular -s, the second person -(e)st is ALSO added to PAST TENSE words, either to the past stem in strong (irregular) verbs or AFTER THE -ed in weak (regular) verbs: 

I gave her the horse. Thou gavest her the horse. He gave her the horse.

I made a pie. Thou mad’st a pie. He made a pie.

I wanted to go. Thou wantedst to go. He wanted to go.

This is different from the third person!

*He gaves her the horse. He mades a pie. He wanteds to go. SO MUCH NOPE!

It’s not wrong to add -(e)st to a long Latinate verb in the past tense, but it’s unusual; it’s much more common to use a helping verb instead:

I delivered the letter. (Great!)

Thou deliveredst the letter. (Not wrong, but weird)

He delivered the letter. (Great!)

I did deliver the letter. (Normal if emphatic, or an answer to a question; otherwise, a little weird.)

Thou didst deliver the letter. (Great!) 

And a couple last things:

1.) Third-person -(e)th is mostly equivalent to and interchangeable with third-person -s:

I have a cow. Thou hast a cow. He hath a cow.

I love her. Thou lovest her. He loveth her.

I do not understand. Thou dost not understand. He doth not understand.

HOWEVER! Third-person -(e)th, unlike -s but like -(e)st, can, sometimes, go on STRONG past-tense verbs:

I gave her the cow. Thou gavest her the cow. He gaveth her the cow.

This never happens with weak verbs:

*He lovedeth her. NOPE NOPE NOPE!

And even with strong verbs, from Early Modern (e.g., Shakespearean) English onward, it’s quite rare. But you will see it from time to time.

2.) In contemporary Modern English, we invert the order of subjects and auxiliary verbs in questions:

Will I die? I will die. 

Has she eaten? She has eaten.

If there’s no auxiliary, we add one–do–and invert that:

Do you hear the people sing? You (do) hear the people sing.

In Early Modern English, this process was optional, and mostly used for emphasis; all verbs could be and were moved to the front of the sentence in questions:

Hear ye the people sing? (Or singen, if we’re early enough to still be inflecting infinitives.)

Do-support was also optional for negatives:

I don’t like him. I like him not.

Thou dost not care. Thou carest not.

She does not love thee. She loves thee not.

3.) Imperative verbs never take endings:

Hear ye, hear ye!

Go thou and do likewise!

Give me thy hand. Take thou this sword. 

4.) Singular ‘you’–that is, calling a singular person by a plural pronoun–arose as a politeness marker; and ‘thou’ fell out of use because it eventually came to be seen as impolite in almost all contexts. In general, once singular ‘you’ comes into use, it is used for addressing

people of higher social status than the speaker

or of equivalent status, if both speakers are high-status

strangers

anyone the speaker wants to flatter

‘Thou’ is used for

people of lower social status than the speaker

family and intimate friends

children

anyone the speaker wants to insult

It is safer to ‘you’ someone who doesn’t necessarily warrant ‘you’ than to ‘thou’ someone who does.

5.) And finally, that ‘ye’? That’s the nominative form of you–the one that’s equivalent to ‘I’ or ‘we.’ 

I  → thou → he/she/it  → we → ye → they

Me → thee → him/her/it → us → you → them

My → thy → his/her/its → our → your → their

Mine → thine → his/hers/its → ours → yours → theirs

Any time you’re using ‘thou’ for the singular, the second person plural– ‘y’all’– declines like this:

ye:  Ye are all a bunch of weirdos.

you: And I love you very much.

your: This has been your grammar lesson.

HUGE thank you to Robert Vernon for accepting my silly meme request. It's finally real.