Question: According to Rav, every Zavah must separate on the day she ceases [seeing], like [Rav said above (68a) about] a Nidah. Why does our Mishnah say that she checked herself on the first day? She already checked herself yesterday, and separated in Taharah!

Answer #1: If he would say [unlike the question assumed] that [a Zavah] need not separate [in Taharah] on the third, this would be fine.

Rebuttal (and Answer #2): We cannot infer so. Even if [our assumption is correct, and a Zavah must separate on day three, and] it suffices to separate on day three according to R. Eliezer (i.e. this helps like checking on the first clean day), it needed to teach that she checked on the first [clean] day, due to the argument of R. Yehoshua and R. Akiva.

Support: This is like [when she checked only on] days one and eight. Rav said that according to R. Eliezer, the same applies, yet it was not taught in our Mishnah, even though according to R. Eliezer it is a bigger Chidush, because then it would not teach the argument of R. Yehoshua and R. Akiva. (Had it taught this case, we could not derive their opinions about when she checked only on days one and seven.)

Suggestion: We can infer from here that she need not count in front of us, since she immerses, even though she was not Mafsik in Taharah, and she did not check also at the beginning of the count!

Rejection #1: Even though a Zavah Ketanah need not count in front of us, a Zavah Gedolah must.

Rejection #2: In any case, it suffices that she checked on the day of counting and found herself Tehorah;

If checking at the end (i.e. not at the beginning) of the day did not count [towards her seven clean days], there would be no solution for Benos Yisrael unless they checked at dawn, for counting at night is not counting.

Explanation: [The widow is disqualified] from Terumah, and Rashi (Yevamos 15b) explained that "Ki Sihyeh l'Ish Zar", from which we learn that once she had Bi'ah with someone forbidden to her, she is disqualified from Kehunah, discusses a widow to a Kohen Gadol;

It does not discuss Machzir Gerushaso (one who remarried his divorcee after she married someone else), for he is not a Zar me'Ikara (he was not forbidden to her from birth).

Question #1: Granted, we cannot learn from this verse. In any case she should be Mischaleles (profaned) due to the Kal v'Chomer itself!

Question #2: In Yevamos (44a), R. Yehoshua learns that [the son of] Chayavei Kerisus is disqualified from this Kal v'Chomer of a widow to a Kohen Gadol, and the Gemara asks that she herself should be Mischaleles from this Kal v'Chomer, and be forbidden even to Terumah!

Answer: Since Chalalah results only from Isurei Kehunah, we cannot learn in general to disqualify the woman herself with an Isur of Kehunah. (Maharam - she can become forbidden, but she cannot get the full status of a Chalalah. Below, Tosfos suggests that we learn an Isur from a Tzad ha'Shavah, but not the full status of a Chalalah.)

"Chalalah" is extra there (the Torah already forbade her to [all] Kohanim. There was no need to write that she is forbidden to a Kohen Gadol) to teach that Chalalah results only from Isurei Kehunah, i.e. the Pesul of the woman who was Kosher and became profaned.

Implied question: An Amoni or Mo'avi convert disqualifies [a Bas Yisrael with whom he has Bi'ah], even though this is not an Isur Kehunah!

Answer: There we expound from a verse, but we cannot learn from a Kal v'Chomer.

Question #1: We should learn a Tzad ha'Shavah from a widow to a Kohen Gadol, and a Nochri or slave [who had Bi'ah with a Bas Yisrael], or from [a Tzad ha'Shavah of] a Chalal and (one of the following who had Bi'ah with a Bas Yisrael -) a Nochri or slave or an Amoni, Mitzri or Edomi convert, that she herself is Mischaleles!

Implied question: You cannot learn from a widow to a Kohen Gadol, or a Chalal, for they are Isurei Kehunah!

Answer: We can say that a Nochri or slave is Yochi'ach (proves that the law does not depend on this). They are not Isurei Kehunah, and they disqualify through Bi'ah!

Implied question: You cannot learn from a Nochri or slave, for they are forbidden to marry Yisraelim, or because they cannot have Kidushin!

Answer: A widow to a Kohen Gadol, or a Chalal, is Yochi'ach. What is common to them is that they are unlike the majority of Klal Yisrael, and they disqualify through Bi'ah. Also Machzir Gerushaso should disqualify her through his Bi'ah!

Strengthening of Question #1: So R. Yehudah shows that a convert disqualifies through Bi'ah, in Kidushin (78a) through a Tzad ha'Shavah of Isurei Kehunah and another matter with it, i.e. a Chalal or Mitzri Rishon (a Mitzri convert. He and his children (who are Sheniyim) are forbidden to Yisraelim, and the third generation is permitted).

Question #2: We can also ask why we require in Yevamos (68a) the verse Ki Sihyeh l'Ish to teach that an Amoni or Mo'avi convert disqualifies through Bi'ah. We should learn from a Tzad ha'Shavah from a Chalal and a Nochri or slave, like I explained!

Answer: Since from a Nochri or slave the child is a Mamzer, we cannot learn from them to disqualify through Bi'ah, since they are so stringent that the child is a Mamzer;

Only R. Yehudah learns in Kidushin (78a) that a convert disqualifies through Bi'ah, from a Mitzri Rishon, even though [a Mitzri Rishon's] daughter is forbidden to Yisraelim. Rabanan disagree with this reasoning.

Support: This answers another question from Yevamos (77a). It says that the daughter of an Amoni convert is Kosher to Kehunah;

The Gemara asks that we should say that she is Pasul, due to a Tzad ha'Shavah of a widow to a Kohen Gadol and a Chalal, and answers that we can challenge it. There is an Aveirah in both sources. (Even though a Chalal's Bi'ah is permitted, he results from an Aveirah.)

Question: Why don't we say that a Mitzri Rishon is Yochi'ach? (No Aveirah was involved, yet his daughter is forbidden.) This is how R. Yehudah learns in Kidushin (78a)!

Answer: Since his daughter is disqualified to Kahal [Yisrael], we do not learn from him a Pesul Kehunah. Only R. Yehudah does.

Question: According to the opinion (Yevamos 45a) that if a Nochri or slave had Bi'ah with a Bas Yisrael, the child is Kosher, if so we should learn that she herself is Mischaleles, like I explained!

Answer: We do not learn from Isurei Kehunah and another matter with it. Only R. Yehudah does, like we find in Kidushin. Rabanan disagree with this reasoning.