What is the meaning of "Lo Yamush"?
R. Yishmael (Menachos 99b): It is a Mitzvah to learn Torah all one's days, even if he has already learned the entire Torah. 1
R. Shmuel bar Nachmani (Menachos 99b): This is a Brachah [that Torah will not depart from you and your seed. Refer to 1:8:1:1*.]
Malbim: It is to engage in learning the laws of the Torah and the Mitzvos.
Radak: It seems that it was a Mitzvah to Yehoshua, and to every person, until he learns the entire Torah. Afterwards, he learns Chachmos.(Ben Dama suggested so, but R. Yishmael rejected this!
What is "Sefer ha'Torah ha'Zeh"?
Rashi: It is Sefer Devarim, which was in front of him.
What is the meaning of "v'Hagisa Bo"?
Rashi: It is contemplation. Every Higayon in Tanach is in the heart
Why does it say Tatzli'ach (Hif'il, i.e. causative)? A person cannot cause his success!
Radak: If one guards the Mitzvos, he will be rewarded with success. It is as if he caused his success.
Rashi (verse 7) writes that Taskil is to succeed. Here it says Tatzli'ach (success) and also Taskil!
Radak: Here, Yonasan translated Taskil as Tichshat (an adornment), which is close to success.
Malbim: Maskil is one who always chooses good methods that lead to the desired ends. Matzli'ach is one whose Mazel enables him to achieve the desired end. Through learning Torah, Hashem will improve your Mazal, and you will be Matzli'ach in worldly matters, and Maskil in spiritual matters.
Why does it say "k'Chol ha'Kasuv Bo"?
Malbim: Also traditions, after investigation they are found written and hinted at in the written Torah through the words and ways of expounding the Torah.