Who wrote Romeo and Juliet?

Shakespeare, around 1595, and he made almost none of it up. The story was already old: an Italian novella of the 1520s, retold by Bandello, translated into French, and then into a long English poem by Arthur Brooke in 1562 which Shakespeare clearly worked from directly. Nearly all his plots are borrowed, which was entirely normal and not considered a weakness.

What he changed was the pacing and the language. Brooke's version spans nine months; Shakespeare compresses it into four days, which is why the play feels like something falling rather than unfolding. Brooke also framed it as a warning against disobedient children, and Shakespeare's prologue instead blames the parents' feud in the first eight lines.

The vocabulary claims made for him need trimming. He is often credited with inventing 1,700 words, and the real figure is far smaller - the count comes from the Oxford English Dictionary recording his as the earliest surviving written use, which mostly means he was the first popular writer to put down something people were already saying, and that the OED's early readers combed his work far more thoroughly than his contemporaries'. He was a magnificent user of English rather than its inventor.