Rambam - 1 Chapter a Day

Sewing refers to stitching the garment in a non-professional manner in which the sides of the tear are not evenly joined. Mending it refers to repairing it in a manner that – to whatever degree possible - the tear cannot be recognized.

For his parent’s passing is a loss that cannot be restored.

Shulchan Aruch (Yoreh De’ah 340:15) quotes Mo’ed Kattan 22b which states that a woman may sew her garment immediately because of her honor. Implied is that even when it will not be immodest for her to walk around with the tear - because she is wearing undergarments - she may mend her clothes for women feel extremely uncomfortable wearing torn garments.

In Hilchot Talmud Torah 5:9, the Rambam writes:
When his master dies, he should rend all of his garments until he reveals his heart; he may never mend them.
When does the above apply? With regard to one’s outstanding teacher from whom one has gained the majority of his wisdom. If, however, one has not gained the majority of his wisdom from him... he should rend his garments [at his death] as he does for all the deceased for whom he is obligated to mourn. Even if he learned only one thing from him... he should rend his garments because of’ his [death].
See also Halachah 11 and notes.

The term nasi refers to the head of the Sanhedrin. It is also used in reference to a king. From the fact that the Shulchan Aruch quotes this law, one may conclude that it also refers to an outstanding Torah leader in any age.

The Sage who is second in stature in the Sanhedrin.

I.e., the majority of the Jewish people (Rashi, Mo’ed Kattan 26a).

Although the standard published texts state “the mountains of Judah,” from Halachah 10, it is obvious that this is printing error.

This ruling definitely applies in the present age. Nevertheless, we find that there are many - even scholars of distinguished piety and knowledge - who do not rend their garments every time they visit Jerusalem beyond a 30 day interval. While an adequate halachic explanation for this lack of observance is not available, one of the ways to avoid the difficulty is to used borrowed clothes when visiting these places or to come on the Sabbath when it is forbidden to rend one’s garments.

In which instance the tear is not noticeable on the surface of the garment.

Since it was mended in a manner in which the tear was not noticeable, it is as if it was being rent for the first time.

For the tear should remain distinct.

I.e., the original owner of the garment who tore it in mourning.

For otherwise, the seller will have deceived the purchaser. Moreover, if a person is sold a torn garment, he may not mend it until he clarifies that it was not torn in mourning. Similarly a torn garment may not be sold to a gentile, for we fear that he will mend it [Tur; Shulchan Aruch (Yoreh De’ah 340:19)].

When describing Elisha’s conduct a s Elijah was being taken to heaven. Although Elijah ascended into heaven alive and did not die, for Elisha, it was as if he was dead (Mo’ed Kattan 26a).

As one is obligated to do at the death of a parent, but not at the death of another relative (Chapter 8, Halachah 2).

When quoting this law, the Shulchan Aruch (Yoreh De’ah 340:36) deviates slightly from the Rambam’s wording, stating: “One must rend his garments because of an unfavorable report, e.g., that the majority of the community gather for a battle, and one hears that they fled before their enemies. [This applies] even if only a minority were slain.” The Ramah adds: “The same law applies if they were taken captive.”

Since the verse states “for” before the object of each clause, we can infer that each of these tidings is - in and of itself - sufficient to warrant the rending of one’s garments (Mo’ed Kattan 26a).

With regard to Ravshakeh's - the agent of King Sannecherib, but according to tradition (Sanhedrin 60a) an apostate Jew - blasphemy of God's name during the siege of Jerusalem. See also lsaiah, chs. 36 and 37, which describes this event.

For Chizkiyahu rent his garments upon hearing the report of the blasphemy from these men (11 Kings 19:1). See Hilchot Avodat Kochavim 2:10 which also describes the obligation to rend one’s garments in this instance within the context of its discussion of the prohibition of blasphemy.

Who relate how the blasphemer cursed God’s name in court, repeating his words; see Hilchot Avodat Kochavim 2:8.

Sanhedrin 60a derives this from the above narrative which relates that Chizkiyahu rent his garments, but the others did not. Why didn’t they? Because they had rent them when they heard the blasphemy.

See also Shulchan Aruch (Yoreh De’ah 340:37) which states that the same law applies with regard to a scroll of the Books of the Prophets or tefillin which are burnt.

Similar laws apply if a scroll is torn or erased in a forceful manner. For one is mourning the desecration of God’s name (Siftei Cohen 340:56).

In its description of the burning of the scroll of prophesy which Jeremiah had prepared by King Yehoyakim.

The people had journeyed to bring sacrifices in the Temple only to find that it had been destroyed by the Babylonians.

See Hilchot Ta’anit 5:16-18 where the Rambam also discusses this obligation.

Man or woman [Kessef Mishneh; 5’hulchan Aruch (Yoreh De’ah 340:5)]. See Turei Zahav 340:2.

Mo’ed Kattan 25a explains that the passing of any person is comparable to the burning of a Torah scroll for both the Torah and the soul are called “the lamp of God” (Rashi). In his Kessef Mishneh and his Shulchan Aruch (loc. cit.), R. Yosef Karo cites the ruling of the Maharam of Rutenberg who states that this applies even to a Jew who transgresses, as long as he does not do so in conscious rebellion against God.

I.e., a person who is reputed to observe mitzvot, eschew prohibitions, and conduct himself in a moral manner [Shulchan Aruch (Yoreh De’ah 340:6)].

Even those people who were not present at the time of the person’s death. If they encounter the corpse before its burial (or hear about the person’s passing according to Siftei Cohen 340:12), they must rend their garments [Shulchan Aruch (Yoreh De’ah 340:6)]. The Ramah, however, quotes the Hagahot Maimoniot which state that although one must mourn over the death of every Jew, the custom is that only those present at the time of a person’s death are required to rend their garments.
See also Shulchan Aruch (loc. cit.) and Siftei Cohen 340:13 who discuss whether this obligation is incumbent on Torah scholars or not.

In this context, Rashi (Mo’ed Kattan 22b) defines this term as referring to a scholar appointed by the community to answer halachic questions. The Shulchan Aruch (Yoreh De’ah 340:7) states that it refers to a person who can provide an answer for any question in Torah law, regardless of the subject matter.
Again citing the Hagahot Maimoniot, the Ramah states that this custom is also not followed in the present age unless the sage who passes away is one’s teacher.

Showing the same extra measure of respect one shows for his parents (Chapter 8, Halachah 3; Radbaz). As mentioned in the notes to that halachah, the Ramah (Yoreh De’ah 340:17) states that at present, the prevailing custom is not to uncover one’s arm under any circumstances. This also applies with regard to the instances mentioned in the following halachot.

In his Kessef Mishneh, Rav Yosef Karo explains that this practice was instituted so that the students would eulogize and mourn over the sage. He also quotes the Tur who clarifies that the intent is not, heaven forbid, that his students suspend their studies, but rather that after the eulogy, they should study at home instead of collecting in the house of study. Rav Yosef Karo also quotes these concepts in his Shulchan Aruch (Yoreh De’ah 344:18).

I.e., even individuals who are not worthy of the title, sage. In this instance as well, the Ramah (Yoreh De’ah 340:8) advises leniency and does not require the tearing of garments.

I.e., before the burial, and certainly before the passage of the seven days of mourning in contrast to the requirements mentioned in Halachah 1.

The Tur and the Shulchan Aruch (Yoreh De’ah 242:25) state that one need not mourn an entire day; a portion of the day is sufficient.

The Shulchan Aruch (Yoreh De’ah 340:7) states that this applies only when the report comes within 30 days of the sage’s death.

The Radbaz (in his gloss to Halachah 11) explains that uncovering one’s left arm is a greater sign of mourning than uncovering the right, because the heart is closer to the left.

I.e., the congregants do not pray in the synagogue, but in the home of the deceased. Nevertheless, on the Sabbath (and on Monday and Thursday), when they must read the Torah, they gather in the synagogue for that purpose alone [Kessef Mishneh, Shulchan Aruch (Yoreh De'ah 344:18)]. The Radbaz (and Rashi, Mo’ed Kattan 23a) differ and maintain that communal prayer should be suspended at this time.

I.e., the suspension of the houses of prayer and study is not to relax one’s religious obligations, but instead to impress people with the obligation to mourn (Mo’ed Kattan 23a).