What exactly does it mean when it writes "v'Se'ar ba'Nega Hafach Lavan"?
Rashi (citing Toras Kohanim): Initially the hair was black, and it turned white in the middle of the Nega. The minimum of 'Se'ar'is two hairs. 1
See Sifsei Chachamim.
Why is a Nega deeper than the skin?
Rashi: [It is not actually deeper. Rather,] it appears deeper, since (bright) white always 1 appears deeper than the skin, like the spot on which the sun shines appears deeper than the shade (Shevu'os, 6b).
Ramban: It is deeper only if there are white hairs in it. 2
Why does the Torah refer to the Nega by different names; sometimes it writes "Tzara'as Hu", sometimes "Tzara'as Hi", sometimes "Tzara'as Noshenes Hi" and sometimes "Nega Hu"?
Rashi (on Pasuk 8): "Tzara'as" is feminine, whereas "Nega" is masculine. 1
Seforno: Like all illnesses, Tzara'as comes in stages - the beginning, the middle and the end, and the same applies to when it heals. In the initial stage, it refers to it as "Nega", then as "Nega Tzara'as", later still, as "Tzara'as" and finally, as "Tzara'as Noshenes". And when it has healed, it refers to it first as "Nirpa ha'Nega" and as "Nirpa ha'Nesek". 2
What are the connotations of "v'Timei Oso"?
Rashi and Ramban: The Kohen declares him [Muchlat, i.e. totally] Tamei. 1 It is a Gezeiras ha'Kasuv that two white hairs are a Siman Tum'ah. What this means is that the white hair is a Divine decree. 2
Why does the period of Hesger (waiting to see whether the Nega will be Muchlat, i.e. totally Tamei) vary? For some Nega'im it is one week, for some two, and for houses it can be three!
QUESTIONS ON RASHI
Rashi writes first that the hair was black, and it turned white in the middle of the Nega, and afterwards he says that the minimum of 'Se'ar'is two. Why did he not explain first 'Se'ar', which is earlier in the verse?
Divrei Eliyahu, Kol Eliyahu: Rashi teaches that "Hafach" means that the Nega made the hair turn color, and not that the hair turned by itself, for the minimum of 'Se'ar'is two. If the verse meant that the hair turned by itself, it would have said "Hafchu".