Why did the workers bring the Mishkan to Moshe in pieces? Why did they not erect it first?
Rashi: Because, due to the weight of the planks, they were unable to erect it. 1 And, because Moshe did not have a hand in Meleches ha'Mishkan, Hashem left it for him to erect. Even then, he only (pretended to) put it up with Divine Assistance. 2
According to the Gemara in Shabbos, 92b, whereas the people were unable to erect the Mishkan due to its size; Moshe was able to, since he was ten Amos tall (Sifsei Chachamim).
Hashem told him to make the movements of erecting the Mishkan, and the planks would erect themselves - which explains why the Torah says later (in Pasuk 17) "Hukam ha'Mishkan", implying that it erected itself.
What is the definition of "the Mishkan", "the Ohel" and "Kol Keilav", respectively?
Ramban and Seforno: "The Mishkan" refers to the bottom set of curtains 1 (consisting of threads of gold, Techeiles, purple and crimson wool, and twined linen) "the Ohel", to the curtains of goats-hair 2 (and "Kol Keilav", to the Mishkan's accessories [Seforno]). 3