Rashi: The Sarim of Ever ha'Nahar - "Tatnai Pachas Avar Naharah u'Shsar Boznai u'Chnavas'hon" (Ezra 5:3) - you ceased the work until now. From now, you will be a plain in front of Zerubavel. You have no authority and grandeur over him.

Radak: This refers to Sanvalat and his colleagues, who prosecuted [against building the Mikdash]. If you will be like a great mountain, you will be a plain in front of Zerubavel. You cannot oppose him to cease the work.

Malbim: The verse depicts that after a long time, the foundation rock sunk into the depth. When the time of the future Binyan comes, it will be swallowed in this big mountain, and they cannot extract it. The verse commands the mountain to flatten, so they can extract the rock.

Rashi: The architect will take the cornerstone in his hand, for he is the first of the builders. They build everything according to his command - a beautiful building.

Radak: Just like [Zerubavel] took out the cornerstone when the building began on the 24th of the ninth month, so he will finish the work.

Rashi: All will laud the rock's grace - how beautiful is the building built via this rock. "Teshu'os Chen" is like "Teshu'os Noges" (Iyov 39:7), "Teshu'os Mele'ah Ir Homiyah", "Kol Sha'on ¹ " (Yeshayah 22:2, 66:6) - all of them refer to making sound heard.

Radak: When Zerubavel put the cornerstone in the building, it was amidst clamor and a multitude, and the entire nation said "Chen Chen Lah" - this rock has grace from Hashem. It is doubled to strengthen the matter, like "Shalom Shalom la'Rachok..." (Yeshayah 57:19).

Malbim: When the mountain flattens so they can extract the rock, all will clamor about the rock "Chen Chen" - it will have grace in everyone's eyes, unlike in Bayis Sheni, when it was not pleasing to the elders who saw Bayis Rishon; they cried greatly, like it says in Ezra (3:12) and Chagai (2:3).