Bearing in mind that those aren’t actually Caesar’s words but Shakespeare’s, or some earlier playwright’s, there are some theories.
“One theory states that the historic Caesar adapted the words of a Greek sentence which to the Romans had long since become proverbial: The complete phrase is said to have been “You too, my son, will have a taste of power”, of which Caesar only needed to invoke the opening words to foreshadow Brutus’ own violent death, in response to his assassination.”
But whether or not the real or Shakespearean Caesar actually said “Et tu, Brute?”, he was correctly using the vocative case of Brutus, “Brute” (rhymes with bootay). This is the proper case to use when addressing someone, and shows Caesar was a stickler for grammar even while being stuck.
Oure Hooste saugh wel that the brighte sonne
The ark of his artificial day hath ronne
The ferthe part, and half an houre and moore,
And though he were not depe ystert in loore,
He wiste it was the eightetethe day
Of Aprill, that is messager to May;
I wonder if the poor unsuspecting Italians were experiencing something like that: some of the words are familiar, but with enough weirdness to lose the message.
Our host saw well that the bright sun
The arc of its artificial day has run
The fourth part, and half an hour and more
And though he was not deeply steeped in lore
He knew it was the eighteenth day
Of April, that is messenger to May;
You mean you don’t read Chaucer in the original? What are you doing with your life?
“Stickler for grammar” is overstating the point a bit. The vocative case was used in Latin, Greek, and probably every other Indo-European language spoken in Caesar’s time. It simply wouldn’t occur to him not to use the correct form of the word that the sentence demanded - anything else would just sound grating to any contemporary. Source: my native tongue has seven cases for nouns, including the vocative; trying to use the nominative in place of the vocative would come across as “caveman speak”.
I may have been a little facetious in writing that. My main point was to explain to anyone who didn’t know why the word is “Brute”, not “Brutus”. Obviously that’s not you.
You’re right that Caesar, in great distress, would use the form of grammar that he had always used. If I were approached by a gang of senators I would certainly say, “What are all those knives for?” rather than, “What are all them knives for?”, because that’s the way I have spoken.all my life.
I totally understand the need to support native languages, but Finnish (and Basque, as they are linguistic partners) are such ancient unique languages that on a practical level, everyone who speaks those languages really does need to be fluent in a more global language, too.
(Aside: I’m somewhat confused by the purpose of な here as it’s been taught as an adjectival marker which obviously isn’t the case here. I’m guessing that combined with the typically feminine informal interrogative の is making it into something kind of like, “that’s right, isn’t it?”)
I have long ago lost track of the original author but I saved this snippet from an otherwise extremely dry linguistics text that was part of the discussion around a book called ボクハ ウナギダ (Boku wa Unagi Da; lit. “I am the eel”), the sum of which is that “da” in Japanese performs functions that aren’t directly parallel to the copula in other languages, and I thought I would share it because it’s one of my favourite things ever written:
"Only Daniels, while making a negative point - i.e. pointing out the “da” that are not “da” - does not give a positive explanation of what the “da” is in unagi and identify sentences. For “da” that are not “da” may well exist, but so do “da” that are “da”.