is "weren't you..." considered grammatically correct? Because expanded, it would translate to "were not you..."
It's absolutely considered grammatically correct. Remember, languages change over time, and abbreviations being added to languages is normal, sometimes leaving the abbreviation in common usage but the expanded form not in common usage.
This is one of those cases. The abbreviated usages are correct and very common:
- Wouldn't you...?
- Weren't you...?
- Won't you...?
- etc.
whereas their expanded forms, though still technically grammatically correct, are NOT common:
- Would not you...?
- Were not you...?
- Will not you...?
Update:
Let me also add there's some really good comments and additions here, and I should note that the correct archaic form (think Shakespearean speak, or King James version of the Bible talk) is to use subject-verb inversion for interrogative sentences, which I'm pretty confident we adopted from French (see: https://www.lawlessfrench.com/grammar/inversion/), since tons of English comes directly from French. In other words, the correct archaic interrogative order would be Verb + Subject + negative, like this:
- Would you not...?
- Were you not...?
- Will you not...?
Though in the imperative positive (or interrogative with inflection) it is Subject + Verb, like this:
- You would...(./!/?)
- You were...(./!/?)
- You will...(./!/?)
However, the pronouns and verbs aren't correct for the archaic form, as "you" is actual the formal 2nd person pronoun, and "thou" is the informal (familiar, but archaic) 2nd person pronoun, and we got rid of the informal verbal conjugations and reduced English verbal conjugations down from 3 forms to 2 in modern-day English (as opposed to 6 verbal conjugations of Romance languages like Spanish and French), so the real archaic English negative expanded forms would be something more like this (I could be off a little on the verbal conjugations, as I could use some references myself to verify these--this is just from my memory from when I lived in the 1500 and 1600 hundreds):
- Wouldst thou not...?
- Wast thou not...?
- Wilt thou not...?
...and for kicks (note to non-native English speakers: meaning: "...and for fun"):
- Knowest thou not...?
- Wist thou not...? <-- my favorite; it also means "You didn't know...?", from the ancient verb "to wit", of course. :)
Notice the archaic 2nd person "thou" pronoun instead of the modern day (technically formal "you"), and the modified verbal conjugations for each verb to make them properly correspond to the informal/familiar "thou" pronoun, instead of the formal singular/plural "you" pronoun.