פירוש על יהושע 1:2

Moshe, My servant is dead. But if he were alive,⁴According to Rashi, this is connected to what follows: Since he is dead, you must arouse yourself and cross this Jordan. I would have wanted him [as the leader].⁵According to Metsudas Dovid, Hashem said that if Moshe was still alive, you would be unable to cross the Jordon because of the decree preventing him from crossing the Jordon. See Bamidbar 20:12; Ibid 27:12,13. Our Sages explain [that this passage is a reference] [to the three thousand laws that were forgotten] during the period of mourning for Moshe. Yehoshua then came and asked.⁶When the Bnei Yisroel realized that they had forgotten so many laws of the Torah, they came to Yehoshua and asked him to teach them the laws, but he too had forgotten many of them. He wanted Hashem to reveal those forgotten laws to him. the Holy One blessed is He, said to him: “Moshe, My servant is dead, and the Torah is called on his name, which is like telling you that it is impossible.⁷Since the Torah is referred to as תּוֹרַת משֶׁה, the Torah of Moshe, and Moshe is dead, these laws cannot be reenacted. Go and burden them with battles.”⁸God told Yehoshua to get the people occupied with conquering the land. He told him to begin preparations to cross the Jordan and this would cause them to forget about the laws that were lost to them. This refers to the battles of conquest that were required to capture the land.

This means to say, had he still been alive, you would not have been able to cross the Jordan River because it was on him that it was decreed not to cross.

Now that he has died, get up and cross over etc.