John Gill Commentary Jeremiah 39:16

"Go, and speak to Ebed-melech the Ethiopian, saying, Thus saith Jehovah of hosts, the God of Israel: Behold, I will bring my words upon this city for evil, and not for good; and they shall be accomplished before thee in that day." — Jeremiah 39:16 (ASV)

Go and speak to Ebedmelech the Ethiopians
Not that the prophet was to go, or could go, out of prison, to deliver this message to Ebedmelech; but that he should, as he had opportunity, acquaint him with it; either by writing to him, or by word of mouth, when he should visit him; for no doubt he sometimes did, having so great a respect for the prophet.

saying, thus saith the Lord of hosts, the God of Israel ;
whom Ebedmelech, though an Ethiopian, served; being a proselyte, and a good man; and therefore would listen unto and believe what came from him.

behold, I will bring my words upon this city for evil, and not for
good ;
meaning the prophecies delivered out by Jeremiah, which Ebedmelech was no stranger to, these should be accomplished; not what promised good, on condition of repentance and amendment; but what threatened evil to the city, and the inhabitants of it, even the destruction of them.

and they shall be [accomplished] in that day before thee ;
signifying that he should live till then, and his enemies would not be able to take away his life; and that he should see with his eyes all that was predicted accomplished, and he himself safe amidst all this.