Jamieson, Fausset and Brown on Jeremiah 18:8

8. their evil--in antithesis to, "the evil that I thought to do."

repent--God herein adapts Himself to human conceptions. The change is not in God, but in the circumstances which regulate God's dealings: just as we say the land recedes from us when we sail forth, whereas it is we who recede from the land (Eze 18:21; 33:11). God's unchangeable principle is to do the best that can be done under all circumstances; if then He did not take into account the moral change in His people (their prayers, &c.), He would not be acting according to His own unchanging principle (Jr 18:9, 10). This is applied practically to the Jews' case (Jr 18:11; see Jr 26:3; Jon 3:10).