What's the structure of this chapter? Why is it significant?
Moshe received the Torah from Sinai: I say: Since this tractate is not founded on the exegesis of commandments from among the Torah’s commandments, like the rest of the tractates which are in the Mishna, but rather deals entirely with morality and good behavior, and whereas the sages of the nations of the world have also composed books according to the fabrication of their hearts, concerning moral paths, how a person should behave with his fellow; therefore, in this tractate the tanna began "Moshe received Torah from Sinai," to tell you that the principles and morals which are in this tractate were not fabricated by the hearts of the Mishna’s sages; rather, they too were stated at Sinai.
כָּל הַמִּצְווֹת שֶׁנִּתְּנוּ לוֹ לְמֹשֶׁה בְּסִינַי – בְּפֵרוּשָׁן נִתְּנוּ, שֶׁנֶּאֱמָר "וְאֶתְּנָה לְךָ אֶת־לֻחֹת הָאֶבֶן, וְהַתּוֹרָה וְהַמִּצְוָה" (שמות כד, יב): "תּוֹרָה", זוֹ תּוֹרָה שֶׁבִּכְתָב; וּ"מִצְוָה", זֶה פֵּרוּשָׁהּ. וְצִוָּנוּ לַעֲשׂוֹת הַתּוֹרָה, עַל פִּי הַמִּצְוָה. וּמִצְוָה זוֹ, הִיא הַנִּקְרֵאת תּוֹרָה שֶׁבְּעַל פֶּה.
BY OUR MASTER MOSES BEN MAIMON
All of the commandments which were given to Moses on Sinai were given together with their oral explanation for, it is said: "And I will give thee the tables of stone, and the Torah and the commandment" (Ex. 24.12.); the Torah, is Holy Writ; and the commandment, its oral explanation. Moreover, He commanded us to observe the Torah by the word of the commandment; thus it is this commandment which is called Oral Torah.
Compiled in Frankfurt, 13th century
When the Holy One came to give the Torah to Moshe, he said over the order of the Readings, the Mishna, the aggadah and the Talmud as it says "And God spoke all these words..." (Shemot 20:1) - even what an experienced student would ask his teacher. The Holy One said to him go and "teach it to the Children of Israel..." (Devarim 31:19) Moshe replied 'Master of the world, you write it down for your children!' Gd said to him, 'I want to give it to them in writing, but it is revealed before me that in the future the nations of the world will rule over them and take it away from them. Then my children will be like the nations!' So Moshe said 'give them the Readings in written form and the aggadah, Mishna and Talmud orally.' "And the LORD said to Moses: Write down these..." (Shemot 34:27) this refers to the Readings, "...for in accordance with these..." (ibid.) refers to the Mishna and Talmud which make the distinction between Israel and the nations of the world.
(ו) דָּבָר אַחֵר, וַיִּתֵּן אֶל משֶׁה, אָמַר רַבִּי אַבָּהוּ כָּל אַרְבָּעִים יוֹם שֶׁעָשָׂה משֶׁה לְמַעְלָה, הָיָה לוֹמֵד תּוֹרָה וְשׁוֹכֵחַ, אָמַר לוֹ רִבּוֹן הָעוֹלָם יֵשׁ לִי אַרְבָּעִים יוֹם, וְאֵינִי יוֹדֵעַ דָּבָר, מֶה עָשָׂה הַקָּדוֹשׁ בָּרוּךְ הוּא מִשֶּׁהִשְׁלִים אַרְבָּעִים יוֹם נָתַן לוֹ הַקָּדוֹשׁ בָּרוּךְ הוּא אֶת הַתּוֹרָה מַתָּנָה, שֶׁנֶּאֱמַר: וַיִּתֵּן אֶל משֶׁה. וְכִי כָּל הַתּוֹרָה לָמַד משֶׁה, כְּתִיב בַּתּוֹרָה (איוב יא, ט): אֲרֻכָּה מֵאֶרֶץ מִדָּהּ וּרְחָבָה מִנִּי יָם, וּלְאַרְבָּעִים יוֹם לְמָדָהּ משֶׁה. אֶלָּא כְּלָלִים לִמְדָהוּ הַקָּדוֹשׁ בָּרוּךְ הוּא לְמשֶׁה, הֱוֵי: כְּכַלֹּתוֹ לְדַבֵּר אִתּוֹ...
Another explanation. "And He gave to Moshe..." (Shemot 31:18) R' Abahu said: all forty days that Moshe was on high, he was learning Torah and forgetting it. He said 'Master of the world, I have only forty days and I don't know a thing!' What did the Holy One do? When forty days were up, He gave Moshe the Torah as a gift, as it says "And He gave to Moshe..." Did Moshe learn the whole Torah? It is written “It is longer than the land and wider than the sea” (Iyov 11) and Moshe learned it in forty days?! Rather, the Holy One taught Moshe general principles (clalim), as it says "When He finished (k'chaloto) speaking with him..."
אמר רב יהודה אמר רב בשעה שעלה משה למרום מצאו להקב"ה שיושב וקושר כתרים לאותיות אמר לפניו רבש"ע מי מעכב על ידך אמר לו אדם אחד יש שעתיד להיות בסוף כמה דורות ועקיבא בן יוסף שמו שעתיד לדרוש על כל קוץ וקוץ תילין תילין של הלכות אמר לפניו רבש"ע הראהו לי אמר לו חזור לאחורך הלך וישב בסוף שמונה שורות ולא היה יודע מה הן אומרים תשש כחו כיון שהגיע לדבר אחד אמרו לו תלמידיו רבי מנין לך אמר להן הלכה למשה מסיני נתיישבה דעתו חזר ובא לפני הקב"ה אמר לפניו רבונו של עולם יש לך אדם כזה ואתה נותן תורה ע"י אמר לו שתוק כך עלה במחשבה לפני אמר לפניו רבונו של עולם הראיתני תורתו הראני שכרו אמר לו חזור [לאחורך] חזר לאחוריו ראה ששוקלין בשרו במקולין אמר לפניו רבש"ע זו תורה וזו שכרה א"ל שתוק כך עלה במחשבה לפני.
Rabbi Yehudah said, "Rav said, "When Moshe ascended to the heavens, he found the Holy One, Blessed be He, sitting and attaching crowns to the letters. He said before Him, "Master of the Universe! Who is staying your hand?" He said to him, "There is one man man who will exist after many generations, and Akiva the son of Yosef is his name, who will in the future expound on every crown and crown piles and piles of laws." He said before Him, "Master of the Universe! Show him to me." He said to him, "Turn backwards." He went and sat at the end of eight rows [of students in Rabbi Akiva's Beit Midrash], and he did not know what they were talking [about]. He got upset. As soon as he got to one [other] thing, his students said to him, "Our teacher, from where do you learn this?" He said to them, "It is a law [that was taught] to Moshe at Sinai." He calmed down. He returned and came before the Holy One, Blessed be He, and said before Him, "Master of the Universe! You have a man like this, and You are giving the Torah through me?" He said to Him, "Be silent. This is what I have decided." He said before Him, "Master of the Universe! You have shown me his Torah; show me his reward." He said to him, "Turn backwards." He turned backwards, and saw that they were tearing his skin with iron combs. He said before Him, "Master of the Universe! Such Torah, and such reward!" He said to him, "Be silent. This is what I have decided."