Berachos, Chapter 1, Mishnah 4. In this mishnah we take up the berachos that surround the recitation of Shema - those berachos that the Sages instituted before the recitation and after it, both in the morning and in the evening.
"Ba'shachar mevarech shtayim lefaneha v'achas l'achareha" - in the morning one recites two berachos before it and one after it:
"Shtayim lefaneha" - two berachos said before the recitation of Shema: the berachah of "Yotzer Or," which concludes "Yotzer HaMeoros," and after it the berachah known as "Ahavah Rabbah" (and in some versions "Ahavas Olam"), which deals with Torah and is said immediately adjacent to the recitation of Shema.
"V'achas l'achareha" - one berachah after the recitation, namely the berachah of "Ga'al Yisrael."
"U'va'erev shtayim lefaneha u'shtayim l'achareha" - and in the evening two before it and two after it:
"Shtayim lefaneha" - the berachah of "HaMa'ariv Aravim" and the berachah of "Ohev Es Amo Yisrael."
"U'shtayim l'achareha" - the berachah of "Ga'al Yisrael" and the berachah of "Hashkiveinu."
It is worth noting that "Baruch Shem Kevod Malchuso L'olam Va'ed" is not necessarily said: in Eretz Yisrael it was not the practice to say it, and it is said only under certain circumstances. The mishnah therefore does not deal with it at all, since it is not part of the basic count; and according to Tosafos it is a separate and different institution.
"Achas arukah v'achas ketzarah" - one long and one short:
This expression is the subject of a discussion among the early commentators as to its precise meaning. For the sake of simplicity we will follow the explanation of Rashi: of the berachos said at night after the recitation of Shema - that is, "Ga'al Yisrael" and "Hashkiveinu" - one is long and the other is short.
What is the measure of long and short?
The scope of the content and the topics contained in the berachah:
The long one - "Emes V'Emunah," which folds many matters into itself: the exodus from Egypt, the plague of the firstborn, and the splitting of the sea.
The short one - "Hashkiveinu," which in practice deals with only one topic: the request that we be protected.
"Makom she'amru l'ha'arich eino rashai l'katzer" - where they said to lengthen, one may not shorten:
The mishnah teaches that one may not deviate from what the Sages instituted, in either direction:
Where the Sages said to lengthen a berachah - one is obligated to lengthen it, and one is not permitted to shorten it.
Where they said to shorten - one may not lengthen.
Where they said to conclude, meaning that the berachah ends with a closing formula of a berachah - one may not omit the conclusion.
Where they said not to conclude, such as the berachos recited over mitzvos and over foods - one may not conclude, and it is forbidden to add a closing formula to such a berachah.
In summary: in this mishnah we learned the count of the berachos surrounding the recitation of Shema - two before it and one after it in the morning, and two before it and two after it in the evening; we identified those berachos and saw that "Baruch Shem Kevod Malchuso" stands outside this count; we explained according to Rashi which is the long berachah ("Emes V'Emunah") and which is the short one ("Hashkiveinu") based on the scope of their topics; and we learned the rule that one may not deviate from the formulation of the Sages, neither in the length of the berachah nor in its conclusion.