“You shalt” is not ‘correct’, nor is “Thou shall not”
Putting aside the question of whether conjugation of verbs is a technically a grammatical rule or has some other status, in English, like many other languages, different pronouns call for different verb forms, and mixing them up often produces jarring and unnatural-sounding sentences that most would not consider correct.
For instance the verb 'to be' conjugates as follows in the present indicative:
- I am
- Thou art (obsolete in modern English and replaced by 'you' form)
- He/She/It is
- We are
- You are
- They are
These are sometimes referred to as the first, second, and third person singular and plural forms respectively.
Most verbs now only have two forms in the present tense: the form which is the same as the infinitive, and a he/she/it form (3rd person singular) which has an -s or -es on the end: work/works, pass/passes. A few verbs only have the one form (e.g. can).
However, when the 'thou' pronoun was in common use, many verbs also had a 'thou' form ending in -t, -st, or -est: e.g. shalt, wilt, canst.
Over time the 'you' form supplanted 'thou', and the 'thou' forms also all disappeared. They are recognised largely because they feature in older texts such as the KJV.