Why does the Torah use the expression "Teishev bi'Demei Taharah"?
Rashi: Besides to sit, the word can also mean to remain. 1
Ramban #1: It means that for 33 days, she may be intimate with her husband - even if she sees blood. 2
Ramban #2: It means that during the 33 Yemei Taharah, although she is permitted to her husband, she should 'sit' (on the seat of Nidus 3 on which she sat when she gave birth) and remain forbidden to eat Kodesh or to enter the Beis-ha'Mikdash. 4
As we find in Devarim, 1:46, and in Bereishis 13:18 [Rashi]), and the Pasuk is then coming to teach us that, although the woman is permitted to her husband, she remains forbidden to eat Kodesh or to enter the Beis-ha'Mikdash for 33 days.
See Ramban's explanation (DH 've'ha'Nachon be'Einai' as to the deeper meaning of this term.
A La'av for which will receive Malkos in the event that she transgresses (Ramban).
Sifra, according to Ra'avad: Such blood is Tahor, like Dam Tohar. However, during the period of Nidah, Koshi is not Metaher her.
Why does the Torah insert the word "bi'Demei"?
Rashi and Ramban #1 (citing Ibn Ezra): To teach us that, even if she sees blood, she is Tahor.
Why does blood during Yemei Tohar not make her a Nidah?
Seforno: Because any blood that she sees during the 33 days of Yemei Taharah are not from a current Veses (menstriuation), but from earlier Vestos from the time of her pregnancy, in which case the blood has already spoiled and lost its Tum'ah.
Rashbam (according to Levi, Nidah 35b): Because it is not the blood of Nidus, but blood that flows from an independent source.
Rav (Nidah 35b): Even though the blood is from the same source as Dam Nidah, the Torah decreed that it is Tahor.
What does it mean "b'Chol Kodesh Lo Siga"?
Rashi (from Yevamos 75a): This is an Isur to eat Kodesh; b'Chol includes also Terumah. 1
Moshav Zekenim (from Yevamos 75a): It forbids also touching Terumah. Mizrachi (from 75a) - the Torah called eating 'touching' to equate them; whoever may not eat, he may not touch.
Despite the fact that she immerses at the end of seven days of Tum'ah, and Terumah generally becomes permitted after nightfall, a Yoledes is called a 'Tevulas Yom Aruch, and her seventh day terminates only at the end of her Yemei Tohar - the night before she is due to bring the Korban that will render her completely Tahor (Rashi).
What is the significance of the seven and 33 days of Tum'ah and Taharah of a boy, and the 14 and 66 of a girl?
Ramban (according to R. Yishmael 1 , Nidah 30a): [After the days of Tum'as Leidah, Yemei Tohar extend until 40 or 80 days, which correspond to the days in which a baby boy and girl (respectively) are formed.
Ramban: "Yemei Taharah" actually means pure in the sense of clean, 2 and the Torah decrees seven days of Tum'ah, because the woman generally sees blood throughout the seven days following the birth and 33 additional days, the time it takes for her body to dry; 3 and double for a girl due to the excessive moist and coldness that a girl creates in the womb, which inevitably takes longer to dry. 4
Moshav Zekenim (5): A fetus is formed facing down. Before birth, a girl flips over to face up. This perturbs the womb and causes twice as much pain as birth of a boy.
Moshav Zekenim (7): Amidst labor pains, a woman swears that they will never again be intimate with her husband (Nidah 31b). When a boy is born, all are happy, and after seven days she regrets her Shevu'ah. When a girl is born, she regrets only after 14 days. 5
Why is the Yoledes Asur to Kodesh but Mutar to her husband?
Ramban: The basic distinction is between Kodesh (for which she is forbidden) and Chulin (for which she is permitted) - and her husband falls under the category of Chulin.
What is the significance of the thirty-three days of Taharah of a boy and the sixty-six, of a girl?
Ramban #1 (citing Ibn Ezra): The Pasuk is in fact, talking about periods of forty (7 + 33) and eighty (14 + 66) days, which correspond to the days in which a baby boy and girl (respectively) are formed. 1
Ramban #2: "Yemei Taharah" actually means pure in the sense of clean, 2 and the Torah decrees seven days of Tum'ah, because the woman generally sees blood throughout the seven days following the birth and thirty-three additional days, the time it takes for her body to dry; 3 double for a girl, either because of the reason given by the Ibn Ezra, 4 or due to the excessive moist and coldness that a girl creates in the womb, which inevitably takes longer to dry. 5
The Torah implies that she may enter the Mikdash and eat Kodesh right after Yemei Tohar. She needs to bring a Korban first!
Riva, Hadar Zekenim: The implied Heter is only for eating "Kodesh", i.e. Terumah 1 , but not for entering the Mikdash or eating Kodshim.
Rosh: The Heter to eat Terumah is after the 40 days finish, i.e. at the start of night 41.
QUESTIONS ON RASHI
Rashi writes that "Lo Siga" forbids eating Kodesh or Terumah. What is the source?
Bechor Shor, Sifsei Chachamim (from Yevamos 75a): It is taught with entering the Mikdash, which has Kares. There is no Kares for touching Kodesh, only for eating it.