When did or will Hashem do so?
Rashi: He did, at the time of Galus.
Radak, Malbim: He will, at the time of salvation.
What is the significant of "Ani Hashem Notzerah"?
Radak: This is the opposite of "Haser Mesukaso v'Hayah Leva'er" (5:5; rather, I will guard the vineyard). The Nimshal is, My Hashgachah will be on it constantly.
Malbim: Hashem says so to the vineyard in His song. I will guard it from every external mishap.
What is the meaning of "li'Rga'im Ashkenah"?
Rashi: Bit by bit I give to them to drink the cup of punishments coming to it, lest I pay them at once and finish them off.
Radak, Malbim: This is the opposite of "v'Al he'Avim Atzaveh me'Hamtir Alav Matar" (5:6). I will put on it a spirit of Da'as and Nevu'ah in abundance, like water - "Ki Mal'ah ha'Aretz De'ah Es Hashem ka'Mayim la'Yam Mechasim" (11:9). "Eshpoch Es Ruchi Al Kol Basar" (Yo'el 3:1) refer to Yisrael - "v'Nib'u Bneichem u'Vnoseichem
Why does it say "Pen Yifkod Aleha"?
Radak citing his father: "Aleha" is like "Aleh Zayis" (Bereishis 8:11). I will water it lest even a leaf be missing, and all the more so fruits (Malbim- or a branch) [should not be missing].
Radak: Lest the enemy come upon it, "I will guard it day and night."
Why does it say "Laylah va'Yom Etzerenah"?
Rashi: Night and day, I contemplate (how to punish them without eradicating them).
Rashi citing Targum Yonasan: If not that I pay them for sins that they are used to doing night and day, I would shield them.