"To a given straight line to apply a parallelogram equal to a given rectilinear figure and deficient by a a parallelogrammic figure similar to a given one: thus the given rectilineal figure must not be greater than the paralllelogram described on the half of the straight line and similar to the defect."
That is Euclid's prealgebraic Greek filtered through Heath's Downton-Abbey era English. Perhaps you would have preferred to remain ignorant.
"... the Elements, written around 300 B.C., has survived, though not in an original manuscript written by Euclid himself. The version we use today has been reconstructed from a tenth-century Greek copy found around 1800 in the Vatican Library and from Arabic translations of other lost Greek copies and revisions.... The first printed version ... appeared in Venice in 1482 (Campanus' translation from the Arabic [into Latin]).... A new Greek text was compiled in the 1880s by Heiberg, and that was translated into English in 1908 by Sir Thomas Heath; it is the version to which English speakers mainly refer."
Heath goes into the history at great length in his History of Greek Mathematics and even greater length in his commentary to his translation of Euclid. Some main points: The book of Campanus was followed quickly by a rival translation by Zamberti, who worked from a Greek source, but one that was far from authentic. Contrary to Allen, Pacioli came next, attempting to correct Campanus and refute Zamberti. In 1533 a Greek edition was published by Grynaeus, which became the standard source until 1800. Tartaglia's translation was probably made from the Latin versions of Campanus and Zamberti, not directly from Greek (or Arabic). "All our Greek texts of the Elements up to a century [now two centuries!] ago depended upon manuscripts containing Theon's recension of the work" -- and Heath does not have a good opinion of Theon (Hypatia's father).
The point of all this is that early modern Europe's knowledge of what Euclid actually wrote suffered from, first, repeated translation through at least one intermediate language, and, second, the dubious quality of the surviving ancient Greek manuscripts. Theon and other ancient commentators did not distinguish between making an improved edition of Euclid's book and writing a new book for pedagogical purposes -- it is as if all freshman calculus and physics textbooks today were, or purported to be, new editions of Newton's Principia. Add the facts that inevitably some of the "improvements" were wrong and that there was no reliable way of archiving old versions (through many centuries of war and social decay), and it's clear that one had a mess.
R. W. D. Nickalls, Math. Gazette 77 (1993) 354-359.