Rambam - 1 Chapter a Day

In the previous two chapters, the Rambam explained the prayers recited at various times of the year. He devotes this chapter to the proper times for each of the daily prayers.

In his Commentary to the Mishnah, Berachot 1:4, the Rambam states that any time the Mishnah talks of hours it must be understood as שעות זמניות (seasonal hours, i.e., relative to the length of the day in a particular season). The daylight hours are divided into 12 equal parts, and each seasonal hour is 1/12 of the daylight time—i.e., one twelfth of that time. (See Hilchot Kri’at Shema 1:11.)

Minchah K’tanah in Hebrew. Thus, Berachot 26b states: “When is Minchah Gedolah? From six and a half hours onward. When is Minchah K’tanah? From nine and a half hours onward.”
The terms “greater” and “lesser” refer to the amount of time left from the beginning of the obligation to pray until the service can no longer be recited. Minchah Gedolah includes more time than Minchah K’tanah.

I.e., one may recite the Shemoneh Esreh often during the day as a voluntary service. (See Chapter 1, Halachah 9.)

I.e., only on the eve of Pesach was the afternoon sacrifice offered this early.

The Tur and the Shulchan Aruch (Orach Chayim 234) both advise not to recite optional prayers unless one is sure of his ability to concentrate fully. See the notes to Chapter 1, Halachah 9.

To elaborate on the above: פלג המנחה refers to the point in time halfway between nine and a half hours—the beginning of Minchah K’tanah—and sunset. Since, by definition, sunset occurs at twelve hours, this is ten and three quarter hours, or one and a quarter hours before sunset. As mentioned above, all these times refer to “seasonal hours”—שעות זמניות.

For generally, the Musaf offering would be brought at six hours.

As explained in Chapter 1, Halachah 6, it has become accepted custom to regard the Evening Service as obligatory and recite it every night.

However, one can only recite the Evening Service after פלג המנחה, one and a quarter hours before sunset, when night begins according to Rabbi Yehudah. Also, from the time one recites the Evening Service, one must accept the Sabbath and refrain from any prohibited labors.

Rav Hai Gaon (quoted in the Tur, Orach Chayim 293) mentions that this practice is only allowed for the sake of a mitzvah under extreme circumstances, e.g., one is rushing to bury a corpse or prepare for a wedding. This restriction is placed because doing so implies a certain lack of appreciation of the Sabbath.
Even though one recites the Evening Service including the Havdalah prayers on the Sabbath, one must refrain from doing work until the appearance of three stars.

In practice, when the Evening Service is recited before sunset, the Shema and its blessings are also recited at that time. After the appearance of the stars, the Shema is repeated without its blessings.

I.e., the time for its compensation, since the establishment of תפילת תשלומים was only for the תפילת in proximity to it.

This is the only possibility of such a situation occurring. With the exception of Musaf, all the prayers have a specific time when they should be recited.

Berachot 27a explains that this law is based on the principle that if two obligations must be fulfilled, one a relatively frequent obligation and the other not as frequent, one performs the more frequent obligation first.
Rabbenu Yonah points out that the Minchah mentioned in this halachah is obviously מנחה גדולה, which partially coincides with Musaf, since its time starts at six and a half hours, and the optimum time for Musaf ends at seven hours. He adds that this halachah applies only in a case when one must recite both prayers at the same time. (For example, one wants to eat a meal and must first recite Musaf and Minchah.) However, if he is willing to recite one of them later, he should recite Musaf first and Minchah, later in the day.