What is the first half of the Pasuk coming to teach us?
Rashi: It teaches us that a pool of water that is 'attached' to the ground is not subject to Tum'ah. 1
Rashi: It also teaches us that someone who immerses in a Mikveh becomes Tahor.
Rashbam: This is like the Torah implies above (in Pasuk 34, that only water in a Kli is Mekabel Tum'ah).
What does the second half of the Pasuk teach us?
Rashi and Targum Yonasan: It teaches us that a person who touches Neveilah becomes Tamei even if he is submerged in a Mikveh. 1
Ramban #1: It teaches us that if water that is detached touches the Neveilah, it becomes Tamei.
Ramban #2: It is referring to all the water mentioned here and in Pasuk 34, and teaches that water is subject to Tum'ah when it is detached, but not when it is 'attached' to the ground.
Ramban #3 (citing the Toras Kohanim): It teaches us that the Neveilah of Sheratzim only renders Tamei someone who touches it, but not someone who carries it.
From where do we know that immersing in a Mikveh renders Tahor, for Teme'im other than Tum'as Sheratzim?
Ramban: We learn it from Tum'as Sheratzim with a (Binyan Av). 1
And the reason that the Torah presents it only here is - either because it waited until it had covered all the Tum'os before doing so, or because it is the way of Sheraztim to die in fountains and in pits of water (Ramban).
What kind of Mikveh is Metaher?
Moshav Zekenim, and Ri (in Tosfos Bava Basra 66a-b): Any Mikveh with enough water for a person to immerse in, i.e. three by one by one Amos, even if it is all Mayim She'uvim (that was in a Kli). The Pesul of She'uvim is mid'Rabanan.
R. Tam and Rashbam (in Tosfos Bava Basra 66a-b): A Mikveh of all Mayim She'uvim is Pasul mid'Oraisa. Three Lugim of Mayim She'uvim disqualify mid'Rabanan.
QUESTIONS ON RASHI
Rashi writes that "Yihyeh Tahor" teaches that a Mikveh cannot become Tamei, and it is Metaher one who immerses in it. How can we learn both of these?
Divrei David: "Yihyeh" is extra, to teach that it is Metaher one who immerses in it. We learn that it cannot become Tamei because it was written here, where the Torah discusses receiving Tum'ah.
Rashi writes that one who touches Neveilah becomes Tamei even if he is submerged in a Mikveh. What is the Chidush?
Mizrachi, citing Toras Kohanim: One might have thought that just like the ground saves [what is attached to it] from becoming Tamei, also attached water saves.
Mizrachi: Why did Rashi abandon what it says in Toras Kohanim, and invent his own Kal v'Chomer? Mei Chatas (water on which ashes of Parah Adumah were put) refutes it! It is Metaher Temei'im, but does not shield Tehorim from Tum'ah. In fact, it is Metamei Tehorim! Gur Aryeh - this is not a refutation. Mei Chatas is not Metaher by itself, without Tevilah, therefore it cannot save.
We must say that even when he touches, it is considered a minority that is not Me'akev Tevilah. If not, it is as if he is not in a Mikveh! The second he lets go (from touching the Neveilah), the Mikveh is Metaher him! This Halachah is relevant to one who left the Mikveh before letting go. And even if he lets go in the water, through Tevilah he is a Tevul Yom. (PF)