Albert Barnes Commentary Jeremiah 11:10

"They are turned back to the iniquities of their forefathers, who refused to hear my words; and they are gone after other gods to serve them: the house of Israel and the house of Judah have broken my covenant which I made with their fathers." — Jeremiah 11:10 (ASV)

Their forefathers - literally, "their fathers, the first ones": alluding to the idolatries committed in the wilderness and by the generations whose history is given in the Book of Judges.

And they went after - Rather, indeed! they have walked after other gods to serve them. The "they" refers to the men of Jeremiah’s day.