Having warned against eating all the Tamei species, why does the Torah add "Lehavdil ... "?
Rashi: It is not sufficient to learn about the distinction between the Tahor and Tamei species, one also needs to be proficient in being able to discern the difference between them.
Hadar Zekenim: The verse begins and ends with the letter Lamed. Together, their Gematriya is 60. This teaches that Isur is Batel in 60 parts of Heter. 1
Surely this is a mere Asmachta. Bitul in 60 is not a Gezeiras ha'Kasuv. Rather, Chachamim gauged that almost all Isurim do not give taste to 60 times their own volume, (there are exceptions - refer to 11:11:151:3). One opinion in Chulin 98a-b learns from the foreleg of Eil Nazir. It is forbidden to Zarim, yet it is cooked with the ram (which is 60 times its size) and does not forbid it. (PF)
What is the Torah adding when it writes "Bein ha'Tamei u'Vein ha'Tahor"?
Rashi : One must distinguish between what is Tamei for you, e.g. half the Simanim were cut, and what is Tahor for you, i.e. the majority was cut. 1 The difference between them is as little as a hairsbreadth.
See Ramban DH 'v'Leshon Rashi'.
What is the Torah adding when it concludes "u'Vein Chayah ha'Ne'echeles ... "?
Ramban #1: With reference to "Nefesh" in the previous Pasuk, it commands to distinguish between the creatures that live in the water or on dry land that may be eaten, and those that may not be eaten.
Ramban (from Toras Kohanim): This adds a Lav for Tamei Chayos.
Riva #1: We distinguish between a limb of a fetus that it stuck outside the womb - Shechitah does not permit it, for we apply "v'Vasar ba'Sadeh Tereifah [Lo Socheilu]" - but if it returned inside before Shechitah, it is permitted. 3
Riva #2: We distinguish between aberrations that make an animal Tereifah - e.g. a extra or missing hind leg, and those that do not, e.g. a extra or missing foreleg.
Why does the Torah insert the word "u'Vein ha'Nefesh ha'Chayah"?
Ramban #1: With reference to "Nefesh" in the previous Pasuk, it is a warning to learn to distinguish between the creatures that live in the water or on dry land that can be eaten and between those that cannot be eaten.
Ramban #2 (citing Toras Kohanim): It is coming to add a La'av on eating a non-Kasher Chayah.
The verse mentions distinguishing "between the Tamei and Tahor, between Chayos that you may eat..." - why did it switch the order, and mention first the Heter?
RS"R Hirsh (20:25): Whenever the Torah obligates distinguishing, we must ensure that the two kinds not mix. One may not be Metaher the Tamei or Metamei the Tahor, or to forbid the permitted (Yerushalmi Terumos Sof Perek 5). Also below (20:25), it switches the order like this.