Albert Barnes Commentary Jeremiah 4:28

"For this shall the earth mourn, and the heavens above be black; because I have spoken it, I have purposed it, and I have not repented, neither will I turn back from it." — Jeremiah 4:28 (ASV)

For ... - Because of this doom upon Judah.

I have purposed it - The Septuagint arrangement restores the parallelism:

For I have spoken, and will not repent,
I have purposed, and will not turn back from it.