Sefer Devarim begins with the words, “VeEileh HaDevarim” “these are the things” (2:6), and based on the well-known rule, the Or HaChaim explains that the word ‘these’ distinguishes the Pesukim that follow. He says that Mosheh said these words without a direct commandment from Hashem, while the first four books of the Torah were written under direct commandment. However, why is this distinction necessary?
The Ot Chaim VeShalom disagrees with the Or HaChaim. He says that it is impossible for Mosheh to have been responsible for the writing of Devarim and that he is appalled that the Or HaChaim would even say such a thing. After all, the Gemara explains, “Who is the one who has dishonored the word of Hashem? It is the one who says, ‘the Torah is not from heaven!’” (Sanhedrin 99a). Instead, the Ot Chaim VeShalom explains that Hashem reviewed a series of laws with Mosheh and commanded him to review them with Bnei Yisrael. However, Hashem did not give Mosheh a specific commandment to write down them down. Mosheh took his own initiative and wrote the laws down as Sefer Devarim.
Ramban in Devarim explains that Mosheh left many Mitzvot unsaid until the last days of his life and told these Mitzvot to Bnei Yisrael before they entered the land of Kenaan. Ramban also states that these Mitzvot were ones that applied only to the Land of Israel.
Radvaz severely attacks Ramban. Many Mitzvot that apply only to Israel were already listed in the Torah. Additionally, there are many Mitzvot in Sefer Devarim that apply all over the world and not just Israel. For example, Bircat HaMazon, a mitzvah that applies to every person every day, appears in Devarim. Radvaz concludes that he does not understand Ramban.
The Kli Chemdah explains the position of Ramban. He states that the entire Torah was given to Mosheh on Har Sinai. Nevertheless, Mosheh revealed these laws to the people only when they became applicable. For example, some laws of Nachalah, inheritance, appear only after the appeal of the daughters of Tzelafchad. The laws that apply only to Israel such as Shmitah and Yovel were taught before the Meraglim were sent to check out the land because Bnei Yisrael were about to enter the land and needed to learn the laws. To explain the question of Bircat HaMazon, the Kli Chemdah states that Bnei Yisrael weren’t actually obligated to say this Berachah until they entered the land. Proof to this is that Mosheh had to give specific instructions to Bnei Yisrael to make a Berachah on the Manna, which they ate in the desert.
The Kli Chemdah concludes with his own opinion, which integrates the opinion of the Or HaChaim. Mosheh wrote the first four books of the Torah exactly the way Hashem dictated. Mosheh wrote Devarim in his own words, as the Or HaChaim said, and expanded on the Mitzvot that Hashem told him to discuss. Rabi Yehuda says that we do not use Semichut HaParashiyot, consecutive sections, to prove Halacha in any Sefer except Devarim. This is because the first four books were given directly by Hashem, and we cannot say definitively that one section actually came before another; we cannot possibly know when Hashem commanded which Mitzvah. Thus, because Mosheh wrote it, Devarim is the only Sefer to have logical order. Mosheh arranged Devarim so we can explain it based on placement.
The words at the beginning of the Sefer explain that Mosheh wrote the entire Sefer in his own words. According to its first words, Sefer Devarim is a special book in which Mosheh presents the laws to Bnei Yisrael and explains them the way he sees fit. We can learn a message from this: even if one has explicit instructions, one can use in his or her own editorial to explain the matter to the audience if it is unclear. If Mosheh was able to write a holy book with his own account and explanation of the laws, we should all use the opportunity to explain unclear matters to others.