Why did Moshe have to ask HaShem questions if he was given all the answers on Mt. Sinai?
ב"ה Shalom, There are a few different perspectives to the question. One is that in at least two instances , the Halacha escaped Moshe's mind as Rashi points out. One was in the case of the Midianite women, in which Moshe did not give a ruling how to respond but Pinchas took the action into his hands. (Bamidbar 25:6) In this case, Hashem wanted the credit to go to Pinchas. The second was in regard to the daughters of Tzlofchad who questioned in regard their inheritance of the land since their father had died. (Bamidbar 27:5) . In this case, Rashi says, that the Halacha escaped Moshe because Moshe had taken some credit to himself. And according to another opinion it was because of the special merit of the daughters of Tzlofchad. There is actually a third case, in which Moshe had to consult Hashem, was in regard to Pessach Sheni, for those who were impure for the first Pessach. In this case, it was something that Moshe could not extract from his learning by Hashem because it was not dependent upon reason, but upon tradition which Moshe had to receive directly from Hashem, In this case, Rashi points out the praise of Moshe who as a person of flesh and blood could consult Hashem at any time. (There are other issues of the Mekalel =blasphemer and the Mikoshesh= the person who chopped wood on Shabbat for which Moshe had to consult Hashem) There is also a wider spectrum to answer this question and that is that according to the opinion of Rav Yishmael in the Talmud, Zevachim 115B, Moshe on Har Sinai was only given the general rules but not all the details. In addition, there is the famous story in the Gemara (Menachot 29b) in which Moshe Rabbenu was shown the future and saw R. Akiva was giving a discourse which Moshe Rabbenu was unable to understand andas a result Moshe Rabbenu felt debilitated. When R. Akiva was asked by his students, where he had learned this, R. Akiva answered that this was taught to Moshe on Mt. Sinai. Upon hearing this Moshe Rabbenu was relieved. Although, there is great depth which is to be learned from this story, in its simplest understanding the commentators explain that Moshe Rabbenu was given the foundations of all that is to be learned, however the finer details were meant to be disclosed by later generations All the best