What the texts show. Emanuel Tov reckons the Masoretic Hebrew of Jeremiah to be about 20 percent longer than the Septuagint; Joseph Blenkinsopp puts the Greek at about one eighth shorter; the figures vary with the method of counting. The order differs too. In English Bibles from the Hebrew, the prophecies against the nations stand near the end, in chapters 46–51. In the Greek they follow 25:13, beginning with Elam. Brenton’s Septuagint shows this plainly: its 25:14 is headed with Jeremiah’s prophecies against the nations, starting with Elam.
Two examples you can check. In chapter 10 the Greek lacks the verses of praise found in the Hebrew at 10:6–8 and 10:10 (“there is none like unto thee, O LORD”), and places the line about silver from Tarshish (Hebrew 10:9) inside its verse 5. In chapter 33 the Greek has nothing corresponding to the Hebrew 33:14–26, the promise of a righteous Branch for David and of an unbroken line of Levitical priests. The same promise of a righteous Branch does stand in both texts at 23:5. These differences are not in doubt (Very Strong).
What the scrolls add. Among the Dead Sea Scrolls, 4QJer-b and 4QJer-d are, in places, shorter than the Masoretic text in the way the Septuagint is; in chapter 10, 4QJer-b also arranges the verses as the Greek does. Other Jeremiah scrolls agree with the longer Masoretic form. So the Greek translator was not simply cutting the book; he was translating a Hebrew text of the shorter kind (Strong). Scholars still discuss how far these fragments prove “two editions,” and which came first (Debated), though many think the shorter form is the earlier one.
An old observation. This was noticed long ago. Writing to Julius Africanus in the third century, Origen remarked that in Jeremiah he found “much transposition and variation” between the Church’s Greek and the Hebrew copies.
What the Orthodox Church reads. The Orthodox Church reads Jeremiah in the Septuagint, together with Baruch and the Letter of Jeremiah, which in the Greek Bible stand with Lamentations between Jeremiah and Ezekiel. Translations made from the Hebrew, Protestant and Catholic, follow the Masoretic length and order; Catholic Bibles also include Baruch, with the Letter of Jeremiah as its sixth chapter.
What it does and does not change. The two forms of Jeremiah show how a prophet’s words were gathered and handed on by faithful communities. The shorter Greek text loses nothing essential of Jeremiah’s message of judgment and hope, and the messianic promise of the Branch survives in both. The Church received Jeremiah in the form the Apostles knew, without claiming that the longer Hebrew is a forgery.