Rambam - 1 Chapter 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).