Which Mitzvah is the Torah referring to?
Rashi: It is referring to learning Torah.
Ramban and Seforno: It is referring to the Mitzvah of Teshuvah 1 (which one is obligated to do whilst still in Galus - Seforno).
Ramban: See Pasuk 1 and Pasuk 2. See also Ba'al ha'Turim.
What is the meaning of "Lo Nifleis hi Mimcha"?
Rashi #1 and Seforno: It means that it is not covered (hidden) from you 1 (that it should require Nevi'im to uncover it 2 - Seforno).
Rashi #2 (in Va'eira Sh'mos, 8:18) and Targum Onkelos: It means that it is not separated or detached from you.
Rashi: As in 17:8 and in Eichah, 1:9.
Seforno: As the next Pasuk goes on to explain - "Lo ba'Shamayim hi ... ".
What is the meaning of "Lo Rechokah Hi"?
Ramban, Seforno, Targum Onkelos and Targum Yonasan: It means that it is not far away from you (that it should require distant Chachamim to explain it 1 - Seforno [and even if you are at 'the edge of the heaven' captured by the enemy, the Mitzvah of Teshuvah is close to you - Ramban]).
Michtav me'Eliyahu (1, p.61): It means that every person has some grasp of Emes.
Seforno: As Pasuk 13 explains "ve'Lo me'Eiver la'Yam hi ... ". See Seforno, end of Pasuk 13. Refer also to 30:11:2:1 and to 30:12:151:4.
