כב. וּבְצַלְאֵל בֶּן אוּרִי בֶן חוּר לְמַטֵּה יְהוּדָה עָשָׂה אֵת כָּל אֲשֶׁר צִוָּה יְ־הֹוָ־ה אֶת מֹשֶׁה
‘And Bezalel, son of Uri son of Hur, of the tribe of Yehuda, did all that Hashem commanded Moshe.’ (38:22)
The Torah tells us that Bezalel built the Mishkan according to the word of Hashem. Rashi points out that the Pasuk doesn’t say that Bezalel did what Moshe had commanded him to do, rather, he did what Hashem commanded Moshe on Har Sinai. Bezalel had already figured it all out.
Moshe commanded Bezalel to first build the Kelim (vessels/ furnishings) and then build the Mishkan afterwards. Bezalel says to Moshe that the custom of the world is to first build the house and then the furnishings. Moshe responds that he was right and Hashem had commanded him to first build the Mishkan and then the Kalim. Moshe then says ‘B’zal-al’ you were in Hashem’s shadow. (Gem. Brachos 55a)
All the Achronim ask, how could Moshe forget what was supposed to be done first? Moshe had just taught the Torah in its entirety (the day before, Yom Kippur, was the day he came down with the second Luchos and taught Kol Hatorah Kulah to Klal Yisroel except for the Parsha of the Mishkan since the Mishkan couldn’t be done on Yom Kippur) and now he forgets which was supposed to come first?
One could say that Moshe forgot the Halacha, but it would be very difficult to explain that Moshe forgot such a Pashut Din. We can’t say that Hashem never taught Moshe which one to build first because Moshe says clearly, ‘indeed Hashem commanded me to first build the Mishkan’. So what in the world is going on with this Medrash?
The Or Hachayim answers this question back in Parshas Teruma. The Pasuk says, ‘like all that I show you, the form of the Mishkan and the form of its vessels; and so shall you do. They shall make the Aron etc.’ (25:9)
The Or Hachayim explains (using various Medrashim) that Hashem showed Moshe an image of the Mishkan first, and then all of its vessels afterwards. After showing Moshe the overall blueprint He then went on to explain each one individualy starting with the Aron. So it becomes a bit more understandable that Moshe was a bit confused about which one should come first as Hashem had showed him the Mishkan first, but when showing him how to build everything, he showed him the Aron first.
What did Hashem want to be built first? Moshe wasn’t entirely clear until Bezalel explained the logic of how Man always builds the house first. With Bezalel’s answer Moshe was able to understand what Hashem wanted to be built first and thus Moshe called Bezalel ‘Bzal-al’. (When Moshe said, ‘indeed Hashem commanded me to first build the Mishan’, he meant now I understand that Hashem truly wanted the Mishkan first)
The Levush Haorah explains this episode in a different way. (First he says anyone who says Moshe didn’t know the Pshat is wrong and doesn’t know anything etc.) He says that Bezalel and all Klal Yisroel were confused in how Moshe was teaching them how to build the Mishkan. In Parshas Terumah, which Moshe taught first, Moshe started by saying the vessels are to be built first (25:9). However, in Parshas Vayakel (35:10) he first says that the Mishkan should be built first. All of Klal Yisroel were listening to Moshe and they were confused, which one did he want first?
At that time Bezalel gets up and asks, ‘Moshe which one do you want us to do first, for it makes sense to build the Mishkan first etc’. Moshe then responds by saying that Bezalel is a great Chacham, one who knew the answer before Moshe had told him. Moshe didn’t forget or was confused, he was teaching and before he taught which one was supposed to be built first Bezalel jumped up and answered. Moshe didn’t do it in a confusing way, rather he was telling Klal Yisroel that the Aron was really supposed to be first because the main reason for the existence of the Mishkan was to bring the Shechina down to this world. The Shechina began with the Aron, meaning the Torah. (This is the pashut answer. If you have time I highly recommend seeing the Maskil Ledavid (one of the 11 commentators on Rashi) and the Gur Arye for other explanations on this piece.)