What are the connotations of "Simu Levavchem"?
Rashi: The Navi in Yechezkel, 40:4 includes in this injunction directing one's eyes and one's ears towards the Torah - to banish all other thoughts in face of them.
Rashi: In fact, Yechezkel is referring to the structure of the Beis-Hamikdash, and we can extrapolate that if this is what is required of a physical building, how much more so will it apply to the words of Torah, which are 'like mountains hanging on a hair' - See Sifsei Chachamim.
Which words of testimony is Moshe referring to?
Seforno: He is referring to the warning contained in 'Ha'azinu' - that just as Hashem provided Yisrael with Parnasah without effort to enable them to serve Him, so too, will He destroy them if their sins prevent Him from fulfilling His goal.
What did Moshe mean by "asher Tetzavum es B'neichem ... "?
Seforno: He meant that, at the end of their lives, before they die, they should pass on all these words (Shiras Ha'azinu) to their children in their last will and testament. 1
Seforno: And instruct them to take great care not to corrupt their ways - just as David commanded his son Shlomo on his death-bed.
To whom does "Lishmor La'asos" pertain?
Seforno: 'So that their children should observe and fulfill all the words of this Torah'.