(Ibid. 28) "And charge (tzav) Joshua": "charging" (tzivui) is prompting to zeal, as it is written (Ibid. 31:7) "And Moses called to Joshua and he said to him before the eyes of all of Israel: 'Be strong and firm'": "Be strong" in Torah, "and be firm" in good deeds.
(Devarim 3:28) "for he will pass over before this people": If he passes over before them, they will pass; if not, they will not.
"and he will cause them to inherit": If he causes them to inherit, they will inherit; if not, not. And thus do we find that when they went to do battle with Ai there fell of them some thirty-six righteous men, viz. (Joshua 7:5) "And the men of Ai smote of them some thirty-six righteous men … (6) and Joshua tore his clothing and fell on his face to the ground before the ark of the L-rd until the evening … (7) and Joshua said: Ahah, L-rd, G-d, why did You bring this people across the Jordan … (8) Please, my L-rd, what shall I say now that Israel has turned its back … (10) And the L-rd said to Joshua: Arise! Why are you fallen on your face!'" Is this not what I said to Moses from the very beginning? If he passes over before them, they shall pass; if not, they shall not pass. If he causes them to inherit, they shall inherit; if not, they shall not inherit — and you sent them and did not pass over before them!