What are the implications of the word "Ach"?
Rashi #1 and Seforno: It implies that even though Moshe taught them the Dinim of Tum'ah, there still remained the Dinim of Hag'alah (with regard to metal vessels - Seforno). 1
Rashi #2 (citing the Yalkut Shimoni), Targum Yonasan and Sifri: it comes to preclude utemsils that contain rust, which must be removed before the vessel is Kashered 2 (and utensils that are not unfinished (Gulmei Keilim) - Targum Yonasan).
What is the significance of the metals that the Torah lists?
Rashi (in Rosh Hashanah, 19b): The Torah is confining Tum'ah to vessels made of these six kinds of metals.
In what form is metal subject to Tum'ah?
Targum Yonasan and Sifri: Only when it has been manufactured into a vessel is it subject to Tum'ah 1 (which we learn via a Binyan Av from Tum'as Meis, where the Torah writes in Chukas, 19:18 "ve'Hizah al ... ha'Keilim" - Sifri).
Targum Yonasan: Not in its raw state and not even whe it is formed but not completed. See Na'ar Yonasan and Torah Temmah, citing the Sifri and note 26.
What is the din regarding Tamei vessels that break?