What is this Pasuk coming to teach us?
Seforno: It goes together with the end of the previous Pasuk. Refer to 33:3:4:5.
Torah Temimah #1 (citing Makos, 22b): This teaches us that the Torah contains 613 Mitzvos - since the Gematriyah of Torah is 611 - with reference to the Mitzvos which Moshe taught us, plus 'Anochi' and 'Lo Yih'yeh l'cha', which we heard directly from Hashem.
Sanhedrin, 59a: A Nochri who studies Torah is Chayav Misah - since the Torah writes "Lanu Morashah" - It is our heritage, and a Nochri who studies it is stealing our heritage. 4 Some say that we read Morashah like Me'orasah (betrothed), and it is as if he commited adultery with another man's wife.
To pass on to our children - which explains why the Torah writes "Morashah" and not 'Yerushah'. See also Ba'al ha'Turim. Refer also to 11:19:1:1.
Ramban: It will never be forgotten, and we will never exchange it for another Torah - See Ramban on Pasuk 5.
Ramban (citing the Midrash): Incorporating the many Geirim who come to join us and serve Hashem - See Ramban.
And a Nochri who steals is Chayav Misah.