Executive Summary: "Thou shall not" is WRONG! either "you shall" or "Thou shalt".
As of last night, one of the trending topics on Twitter was the inarticulate "Thou Shall Not".
The sadder thing is that Google has 463,000 results for "Thou Shall Not". Thank the Lord "Thou Shalt Not" still has more (1,990,000), but I feel these data call for a post about English, and this time also in English, as a service to the public.
Guys, saying "thou shall not" is comparable to "I has", or "she have", ok?
In many languages, the verb has to agree with the subject. That means that the verb expresses not only the action, but also the person (first, second, third), number (singular or plural) and in some forms in some languages also the gender of the grammatical subject.
In Modern English, the verb in the present distinguishes only 3rd person singular (ends with s) from all others: I have, you (sg.) have, she has, we have, you (pl.) have, they have. In the past there is no distinction whatsoever: I, you, she, we, you, they – all had. In Hebrew some forms also distinguish gender: e.g. at amart you (sg. f.) said vs. ata amarta you (sg. m.) said. In Russian the past is actually a participial form, that is, it's partly verb and partly nominal (i.e. noun or adjective). So all feminine singulars would end in -a, neuters in -o and masculine singulars with zero: ona byla "she was", vs. on byl "he was", eto bylo "it was" (Dear Lord, forgive my transcription, it's for non-Cyrillic-readers).
The only English verb that distinguishes more than just 3rd sg. is the copula, that is, the verb "to be":
In the present we have I am, you (sg.) are, he/she/it is, and we/you(pl.)/they are.
Believe it or not, Old English had more distinctions than most English speakers can even imagine, both in the verb system and in the nominal system. One day I may write about that.
In any case, not only Old and Middle English but also Archaic New English, has the pronoun thou for 2nd sg., distinct from you, which served for 2nd pl. or as a respectful form for the singular (like usted in Spanish, vous in French, vy in Russian, shomā in Persian). When the subject is thou, the verb agrees with it and ends with -t (art, shalt, hast). When the subject is you, the verb agrees with it and appears in the plural form (are, shall, have).
As it so often happens, the respectful form you took over, and familiar thou was abandoned except in archaic or very high-register (high level of language) uses.The funny thing is, now Thou is considered more respectful than you. The other funny thing is the speakers do feel the need for number distinction in the second person, which gave rise to forms like you guys, y'all, and youse.
I shan't delve into ye, thee, thine and thy right now. I think ye have had enough for one post. To sum it up, the verb should agree with the subject, so if you're using archaic thou, the verb should agree with thou, not with the plural you: Thou art, thou hast, thou shalt ; you are, you have, you shall.
P.S. This lack of agreement occurs also with other verbs, but to a lesser extent: "Thou are": 108,000 results vs. 4,970,000 for "Thou art"; "Thou have": 343,000 results vs. 4,000,000 for "Thou hast".
P.P.S. Come to think of it, this post has added a couple of more results to the search "Thou Shall not", which it denunciates. Maybe at least some of the other results are not so ineloquent after all.
P.P.P.S. I know as a descriptivist I shouldn't use judgmental terminology like "wrong" and "sad", but deep down I'm just a normativist and things like that are literally painful.