What is the meaning of "Pano El Harbe v'Hinei li'M'at"?
Rashi: Until now, you thought to bring much from the field, and you brought little.
Radak: If you will ascend the mountain and bring wood to build the Bayis, the curse will depart from you. However, now you turn to much and it is little - your turning is to diminish, the opposite of your intent, that it will be much.
Malbim: Until now, the curse was because they lacked Beis Hashem. The Navi informs that now that they are commanded to build it, if they will slacken, there will be a curse of Hashgachah. Initially the curse was via Hester Panim (hiding His face). Little grew from much seeds. Now, even though they will grow much and take much to the house, it will be a little.
What is the meaning of "v'Nafachti Vo"?
Rashi: I will strike it with rotting and worms. V'Nafachti is like "Nofe'ach b'Esh Pecham" (Yeshayah 54:16).
Radak: It is like something that is eradicated via blowing with the mouth.
Radak citing Targum Yonasan: I will send a curse in it.
Malbim: I will burn it with My breath.
What do we learn from "Ya'an Mah Ne'um Hashem Ya'an Beisi..."?
Malbim: Until now, I did not arouse them to build. The curse was not a punishment for not building; it cam automatically, via lack of Brachah. The Beis ha'Mikdash is essential for Brachah. If they brought much to the Bayis, they would have much via nature. Now, it will be a punishment, for the Bayis is destroyed and each man desires his house. I.e. he does not strive to build the Beis ha'Mikdash, only his house. Therefore, I will send a curse on the grain in your houses!
