If I’m following all of the technical terms correctly, it appears this feature is not common, even in other Germanic languages. But it’s apparently not completely unique to English; it appears Dutch has the same feature.
Wiki mentions some (apparently!?) mixed-up polite forms in German, like “Das wissen Herr Bürgermeister schon” and “Was geruhen Euer Majestät zu befehlen?”, but English is English and German is German, and furthermore both languages are constantly evolving, so one cannot necessarily copy the usage from one language to the other.
I forgot about this, obviously the “thou” conjugation most familiar to many of us from exposure to the KJV bible.
Originally, though, it was something like “thou bist”. I’ve seen variants of this construction in Victorian era novels, usually spoken by household servants who came from remote parts of England.
I don’t know when “art” supplanted “bist” though.
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As Squink’s chart shows, German does have a few more, although it’s obvious that many cognate words on English decline similarly which is helpful. A particular complication with certain irregular verbs in German is that the third and second person singular present may also have a change of vowel.
Like so much of German grammar, all this becomes easy if you study, listen to, or read the language long enough, but that’s of little use to beginners.
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I am not convinced that it did. ‘Bist’ is a dialect word (from German du bist)heard still in Somerset (mainly Bristol) and Shropshire.
The phrase ‘thou art’ still survives in parts of Northern England, notably Yorkshire, although it is frequently contracted to ‘thart’
If you look at an Old English grammar chart, it is clear that (e)art and bist corresponded to two different verbs with slightly different meanings, so, naively, it seems that the use of one form was eventually pushed aside by the other? But for sure there were and are different dialects of English.
The various conjugated forms of “to be” in English come from three different verbs meaning some different variations on “to be”. They were something like “beon”, “wesan”, and “es”. They got smashed together into the modern conjugated forms of “to be” in English. Really, to understand it, you have to be all the way back to Proto-Indo-European.
“Ye” was originally the nominative form; the verbal context would be the same as with other nominative pronouns.
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In the South in the US, you also have the second-person plural “y’all”… 
(As much as that is one of those “You might be a red neck” words, that’s how language happens.)
“You” was once the second person plural while “Thou” was the singular. Now it does duty for both but retains the plural helping verb.
Just as the plural word “you” began to be used as a singular with “y’all” becoming an unambiguous plural; I’ve read — at SDMB probably! — that “y’all” is now often used as a singular, with “all y’all” now the unambiguous plural!
Old English had two copulas: bēon and wesan.
“The verb ‘to be’ in Old English was suppletive, and used forms from at least three different roots. There were two distinct present stems, for which wesan and bēon were the two infinitive forms. The present *bēon *was used to express permanent truths (the ‘gnomic present’), while *wesan *was used for the present participle and the preterite. They shared the same past tense forms.”
Þū bist was from bēon, while þū eart was from wesan.
Bist—British dialectal: Bristol, West Country, Northern England.
Where bist goin’ (Where are you going?)
How bist? (How are you?)
In post #27, I wrote:
> Really, to understand it, you have to be all the way back to Proto-Indo-European.
I meant:
> Really, to understand it, you have to go all the way back to Proto-Indo-European.
Ah, you had it right the first time. If only we could time travel and hear real Proto-Indo-European spoken and find out if the reconstructions are right or not!
Also, you may wonder about the idea of three words (beon, wesan, es) being combined into a single word meaning “be” in present-day English. That has happened in other cases. The reason why the verb “to go” has some parts of its conjugations looking like “go” and some looking like “went” is because it’s another case of two verbs getting smashed together. “Went” used to be the past tense of “wend”. The past tense of “wend” is now “wended” (and “wend” is now not common), while the form “went” has become the past tense of “go”. “Go” and “be” are very common words in English, and perhaps this is the only case where this smashing together can happen.
Does anyone have other examples of verbs having their conjugations smashed together, in English or in other languages?
English has “be” and “go”.
The Wikipedia overview page lists examples from a few languages; there is also this list (which for English also lists “wreak” in case people use “wrought” instead of “wreaked” or “wroke”)
Thank you, DPRK. I didn’t realize that there was a technical term for it like “suppletion”. i’ve known about this phenomenon for some time, but I didn’t know the name for it.
Off-topic but … interesting?
Discussion of the “are” in “you are” is interesting, but perhaps we should discuss the “you” as well. 
The large number of Norse words in Middle English deserves explanation.
The standard etymologies connect Middle English ye/yu to Old English ge/eow/eow but mightn’t they come from Old Norse ér/yðr/yðr instead? It’s undisputed that they/them derive from Old Norse þeir/þá/þeim.
Many words in Middle English have cognates in both Old English and Old Norse, and are shown as deriving from Old English, but the serious work by Emonds and Faarlund disputes that. Some ME words (although they don’t mention ‘you’) with cognates in both OE and ON may have come from ON.
These linguists think Middle English had an interesting political origin. Prior to the Norman Conquest, the Scandinavians were often predominant (note that the mother() and stepfather of King Edward the Confessor were both Danes); and they retained some form of their Norse language. ( Or rather, mother Emma was the daughter of a Danish woman and a Norman Duke.) “There was [in the 11th century] the same tongue in [the Norse or Danelaw portion of] England as in Norway and Denmark.” But the arrival of the Normans after 1066, plunging both English and Norse into poverty, led to …
I am tempted to find a way to slip some of those miserably neglected verb forms back into conversation. “Thence I yode and never went back” or something.
In the past, “thou” and “Ye” were used as second-person pronouns. “Thou” is singular and “ye” is plural. When time passes, “thou” died and “ye” become “You” and taken for all the purposes. So, “ye”= “You” is always plural.