Rambam - 1 Chapter a Day

See also Chapter 13.

I.e., as explained in Chapter 1, Halachot 13-14, this refers to a person who immersed himself to regain ritual impurity, but has not waited until nightfall of the day of the immersion.

The Rambam says, “a priest,” because, as mentioned in Chapter 1, Halachah 7, this offering need not be brought by the High Priest. He is quoting the wording of Parah 3:1. Indeed, in his Commentary to that Mishnah, he uses that phrase as proof that it does not have to be a High Priest who offers the red heifer.

See the following halachah.

Significantly, in his description of the chambers of the Temple Courtyard in Hilchot Beit HaBechirah, ch. 5, the Rambam does not mention this chamber.

See Hilchot Tum’at Meit 6:2; Hilchot Keilim 1:6.

See Hilchot Avodat Yom Kippurim 1:3. Yoma 2a explains that just as Aaron and his sons were isolated for seven days before the dedication of the Sanctuary, as stated in Leviticus 8:33-34, so too, a High Priest should be isolated for seven days before Yom Kippur and the priest offering the red heifer should be isolated for seven days before performing that service.

Because he was intimate with her while she was in that state and thus contracted impurity (see Hilchot Metamei Mishkav UMoshav 3:1).

See Hilchot Ma’aseh HaKorbanot 5:3.

See Chapter 3, Halachah 1. The Kessef Mishneh explains that since the red heifer is sacrificed outside the Temple Courtyard, one might not show that much care regarding its sacrifice. Hence, an endeavor is made to emphasize that it shares the status of a sin offering.

The Rambam uses the expression mei chata’at, literally, “the water of the sin-offering,” because, as mentioned, the red heifer shares the status of a sin-offering and is referred to in this manner in the Torah and in Talmudic literature.

For he was isolated in the chamber for these three days.

In fact, it could have been the seventh day after he unknowingly contracted impurity, but not the seventh day after his purification began.

Which were instituted, as stated in Halachah 1, to emphasize the sacred nature of the red heifer.

As stated in Hilchot Shabbat 23:8, sprinkling the ashes of the red heifer on a person is forbidden by Rabbinic Law, for by doing so, he is being endowed with the status of purity. Thus it is comparable to completing the fashioning of a utensil.

As stated in the previous halachah. Thus nothing is lost by the omission of the sprinkling on this day.

A certain portion of the ashes of all the red heifers that had been offered previously was preserved, as stated in Chapter 3, Halachah 4. Some of the ashes from each of these red heifers were taken to sprinkle on the priest who would offer the red heifer.

If, however, there are ashes of two or more heifers-even if there are not ashes from all the heifers - the ashes of all those which they possess are used (Parah 3:4).

As Numbers 19:19 states: “And a pure person shall sprinkle on the impure person.”

For this reason, this precaution is necessary. Such a precaution is taken only with regard to the priest who offers the red heifer and not in other instances for the reason the Rambam explained above.

Ordinarily, no matter how deep in the ground a corpse is buried, the impurity pierces through and ascends, making anyone who stands over it impure, as stated in Hilchot Tum’at Meit 7:5. Thus usually, there is a possibility that one has stepped over impurity. If, however, there is empty space of at least a handbreadth by a handbreadth by a handbreadth over the grave, as in the present instance, it intervenes and prevents the impurity from spreading upward.
In his Commentary to the Mishnah (Parah 3:2), the Rambam associates the suspicion of the existence of such a grave with the concept of “impurity likened to the depths,” which he defines (Hilchot Nizirut 6:18) as: “[A human corpse which] no one, not even one at the end of the world knows about.”

Until the age of eight (Tosefta, Parah 3:2). Afterwards, we fear that they might become impure due to a seminal emission.

So that even if the children would ride upon them, the belly of the ox would be broad enough to prevent the child’s legs from hanging over the earth. Even so, they would use the boards, as the Rambam proceeds to explain [the Rambam’s Commentary to the Mishnah (loc. cit.)].

As mentioned in Halachah 1.

A small stream slightly south of the Temple Mount.

They could walk on the Temple Mount, because "the earth beneath it was hollowed out to prevent contracting ritual impurity due to tum'at ohel. Arches above arches were built underneath [for support] (Hilchot Beit HaBechirah 5:1).

In his Commentary to the Mishnah (loc. cit.), when defining this term, the Rambam cites Onkelos’ translation of Genesis 24:20.

In that source, one opinion maintains that the children would not pick up the ashes with their hands from the pitcher, but instead, would go through an elaborate process lest they be impure and thus cause the ashes that remain to become impure. Another opinion — which is accepted as halachah — explains that this process is not necessary.

In Chapter 6, Halachah 2, the Rambam writes that a minor is not acceptable to fill the water for the ashes of the red heifer or to consecrate it. How then are they permitted to perform these activities in this instance? Tosafot Yom Tov (Parah 3:2) explains that since the priest burning the heifer is not known to be impure because of contact with a corpse, sprinkling the water on him is merely a stringency. As such, leniency is shown and a minor is allowed to perform these activities.

See Hilchot Sha’ar Avot HaTum’ah 13:2 which relates that, when immersing, one must have the intent that he is immersing himself for the sake of offering the red heifer.

For, according to law, the utensils and the people are fit to be involved in the second offering without another immersion.