Berakhot 3A

קשיא דרבי מאיר אדרבי מאיר! תרי תנאי אליבא דרבי מאיר.

The previous baraita cited R' Meir’s opinion that the time for the recitation of Shema begins when the priests immerse before partaking of their teruma. In the Tosefta, it was taught that R' Meir holds that one begins to recite Shema from when people enter to eat their meal on Friday. One opinion of R' Meir seems to contradict another opinion of R' Meir. The Talmud responds: Two tanna’im, students of R' Meir, expressed different opinions in accordance with R' Meir’s opinion.

קשיא דרבי אליעזר אדרבי אליעזר!

So too, the opinion of R' Eliezer cited in the Mishnah contradicts the opinion of R' Eliezer cited in the baraita. In the Mishnah, R' Eliezer holds that the time for the recitation of Shema begins with the emergence of the stars: From the time when the priests enter to partake of their teruma, while in the baraita, he states that the time for the recitation of Shema begins when the day becomes sanctified on Friday.

תרי תנאי אליבא דרבי אליעזר, ואיבעית אימא: רישא לאו רבי אליעזר היא.

The Talmud responds: There are two possible resolutions to the apparent contradiction in R' Eliezer’s opinion. Either two tanna’im expressed different opinions in accordance with R' Eliezer’s opinion, or if you wish, say instead that the first clause of the Mishnah, according to which we begin to recite Shema when the priests enter to partake of their teruma, is not actually R' Eliezer’s opinion. Only the second half of the statement: Until the end of the 1st watch, was stated by R' Eliezer.

עד סוף האשמורה.

In the Mishnah, we learned that R' Eliezer establishes that one may recite the evening Shema until the end of the 1st watch. These watches are mentioned in the Bible as segments of the night, but it must be established: Into precisely how many segments is the night divided, 3 or 4? Moreover, why does R' Eliezer employ such inexact parameters rather than a more precise definition of time (Tosefot HaRosh)?