Which matters did he write?
Radak: It is everything from the beginning of the Sefer until the fourth year of Yehoyakim, like it says above (36:1-2) 1 .
Radak (36:2) and Malbim, from Mo'ed Katan 26a: He wrote Megilas Eichah. Malbim - He commanded Yirmeyah to write also what He spoke to him about all nations, i.e. Perakim 46-51 after this. Yirmeyah arranged that in a separate Sefer - "v'Heveisi... Es Kol ha'Kasuv ba'Sefer ha'Zeh Asher Niba Yirmeyah Al Kol ha'Goyim" (25:13). This refers to his Nevu'os here (Perakim 46-51). Therefore, before this Sefer is arranged, it tells what Hashem commanded the scribe Baruch.
Malbim: Above (36:2), it tells that Hashem commanded Yirmeyah to write what He spoke to him about Yisrael and all nations.
