Rambam - 3 Chapters a Day

I.e., a placenta had formed, but it was empty.

I.e., hymeneal bleeding.

I.e., she is under twelve; at this age, it is assumed that even if she menstruates once or twice, it is an irregular occurrence and not the beginning of a pattern (Rashba, as quoted by Kessef Mishneh).

Since three months, i.e., three intervals in which the woman would have been expected to menstruate passed without her doing so, a chazzakah has been established that she will not continue to menstruate according to a usual pattern. Any later menstruation is considered as a new and abnormal event. Therefore she is impure only from the time she discovers uterine bleeding onward.

Since she menstruated three times — even though they were separated by an abnormal number of days — she is considered to have established a pattern is therefore considered like other women.

The fact that she menstruated twice indicates that her physical condition has changed and is similar to that of other women.

E. g., anger, fright, strenuous physical activity, or partaking of certain foods [the Rambam's Commentary to the Mishnah (Nidah 1:6)].

As the Torah relates (Leviticus 1 2:2-5; see Hilchot Issurei Bi’ah 4:5), after the birth of a male child, a woman becomes impure for seven days. Afterwards, she immerses herself to regain ritual purity. For the next 33 days, even if she experiences uterine bleeding, her state does not change and she remains ritually pure. Similar concepts apply after a woman gives birth to a female except that she originally becomes impure for 14 days. Afterwards, she remains pure for 66 days. All bleeding experienced by a woman in these intervals is referred to as "the bleeding of purity."

Until the 24 hours pass, she is not considered as likely to experience bleeding that renders her ritually impure.

Which must be eaten in a state of ritual purity.

Since they are young, we are not certain that they are knowledgeable concerning menstruation and may have experienced bleeding without taking heed of it.

Since she cannot communicate, her intellectual development is impaired. She is considered mentally deficient and unable to take responsibility for her conduct.

Rashi (Nidah 56b) explains that women would have a room where they stayed while experiencing menstrual bleeding. They would bury stained clothes there.

The saliva of a person who has not eaten, beans that have been chewed, urine that has become sour, lye, natron, cumin powder, and bleach, as stated in Hilchot Issurei Bi’ah 9:37.

And thus the garment had never contracted impurity. The Rambam's ruling has attracted the attention of the commentaries, for seemingly, if the stain was a tint and the garment had not contracted impurity, why is it necessary to immerse it? Indeed, Nidah 9:6, the Rambam's source, states that such a garment need not be immersed. The Kessef Mishneh tries to justify the Rambam's ruling, explaining that since the garment was suspected of being impure, it must be immersed before it is considered as pure again. See also the gloss of the Maggid Mishneh to Hilchot Issurei Bi’ah 9:36 which explains that since there is no great difficulty in immersing it, we do not rely on the probability that it was a tint.

I.e., try to pierce it with a needle. At any place where semen is absorbed, it will be more difficult for the needle to penetrate (Rashi, Nidah 61b).

I.e., the garment should be held up to the sun. If it is old, the rays of the sun will pass through the places that are not stained with semen, but not those which are. This would not be effective for a new garment, because its threads will be too dense and will not allow sunlight to pass through even if it is not stained.

Because it is still not evident that the person is bothered by even a trace of the stain.

A measure of volume equal to 86 cc. according to Shiurei Torah and 150 cc according to Chazon Ish.

This measure of blood from a corpse imparts the impurity connected with a corpse (Hilchot Tum’at Meit 2:11).

The 33 or 66 days when, had she immersed, she would have been pure despite this bleeding.

She does not regain purity entirely — i.e., to the extent that she is permitted to partake of sacrificial foods — until she brings the required sacrifices after the days of purity pass.

Both these must be eaten in a state of ritual purity. A person who immersed himself, but is waiting for nightfall may partake of the second tithe (Pesachim 35a), but imparts impurity to terumah if he touches it (Hilchot Sha‘ar Avot HaTum’ah 10:3).

I.e., when she brings the appropriate sacrifices. after bringing these sacrifices, she does not impart impurity to any article, even sacrificial foods that she touches (Kessef Mishneh).

This immersion is significant with regard to relations with her husband. Although she remains ritually impure because of her tzara‘at, she is not forbidden to be intimate with him.

Semen, saliva, and urine.

A woman who experienced one or two days of zivah bleeding, in which instance she is required to experience one spotless day and immerse to regain ritual purity.

I.e., checking to be certain that she does not experience uterine bleeding.

I.e., he is not liable for a sin-offering for unknowingly engaging in relations with a woman in the zivah state.

The laws of zivah apply only in the eleven days that follow directly after the seven days of Nidah. After those eleven days, any bleeding a woman experiences is considered to be Nidah bleeding. Thus if a woman experiences uterine bleeding for the first or second time on the eleventh day of zivah, she could never become a major zavah, because it is impossible for her to experience three consecutive days of zivah bleeding. See Hilchot Issurei Bi’ah 6:4-5; see also the following halachah.

I.e., even if she also experiences uterine bleeding on the eleventh and twelfth days, she will never become a major zavah, because the bleeding of the twelfth day is considered as Nidah bleeding.

As explained in Chapter 8, Halachot 1-3, a zav and a zavah impart impurity if they move an impure article even if they do not touch it. One would think that this would apply retroactively as well. Nevertheless, an exception is made with regard to an earthenware container and it is considered as pure even if a zav or zavah reverts to a state of impurity after touching it (see Megilah 8b).

A zavah, a nidah, and a woman after childbirth. As stated in Hilchot Tum’at Tzara’at 10:6, a person afflicted with tzara'at also imparts impurity to these articles.

Even when one does not touch it, e.g., one is carrying a board on which such an article is resting.

I.e., an article on which a zav sits or lies.

I.e., they become impure and must be immersed in a mikveh to regain purity.

I.e., the verse includes not only the clothes that he is wearing, but also clothes that he touches.

For the verse referring to contracting impurity in this manner does not say: “He shall launder his garments.”

In this instance, impurity is imparted in the following manner: A zav was covered with ten blankets. Even if he did not touch the upper ones, they are considered to have contracted impurity because of this stringency [see the Rambam’s Commentary to the Mishnah (Parah 10:1)]. There is no limit to the number of coverings that contract this impurity. Even 100 blankets become impure. See Halachah 5 for an illustration of this principle.
In his Commentary to the Mishnah (Zavim 4:6), the Rambam cites Nidah 4b which states that the term madaf is rooted in the phrase (Leviticus 26:36): aleh nidaf, “a rustling leaf,” i.e., a movement that is not of substance. Similarly, we find the expression (Berachot 51a): raicho nodaif, “Its fragrance wafted,” which implies having a far-reaching effect.

And we follow the principle (see Nidah 61b, et al), that a Rabbinic safeguard is not instituted to protect another Rabbinic safeguard.

It is as if the zav sat on each one of the articles.

And all those in between.

Although the term used by the Rambam literally means “touch,” we cannot say that this is his intent, because the first clause in the halachah states that the zav need not touch the article to make it impure. Nevertheless, if there is empty space between the objects on which the zav is lying and other bedding below him, the lower bedding does not become impure.

Needless to say, if he touched them, they would become impure, because he touched them.

See Chapter 7, Halachah 8, and Hilchot Keilim 22:6, 25:8, 27:16 for examples of this principle.