Why did Yosef dream about sheaves of grain? And why did their sheaves surround his sheaf?
Ramban, Moshav Zekenim: Because it is in connection with sheaves and produce that they would eventually bow down to him. And their sheaves surrounded his sheaf like subjects surround their king (Ramban).
Da'as Zekenim, Hadar Zekenim: He rose to power through [gathering] grain.
What is the significance of the words "v'Gam Nitzavah"?
Rashi: Yosef's sheaf did not just arise; it stood erect. 1
Seforno: It indicated that his reign would be a long one. 2
Why did the dream of the sheaves come first?
Moshav Zekenim: It was due to [famine, and they needed] sheaves that they bowed to Yosef.
It was fulfilled. The second dream, in which also his parents bowed to him, was not fulfilled. (PF)
Why does it say "v'Hinei" three times?
Peninim mi'Shulchan ha'Gra: V'Hinei begins a matter. The sheaf rising alludes to Moshel (rule by force). Bowing alludes to Melech (willing acceptance). This is why the brothers asked about both of these, and hated him for his dreams (plural), for it was like two dreams.