Why does it say "Al Admas Yisrael"?
Radak: Due to the Churban of Eretz Yisrael. The generation in which it was destroyed, they will say this Mashal.
What is "Boser"?
Rashi: A fruit that did not ripen.
What is "Tikhenah"?
Rashi: They will be set on edge.
What is the meaning of the Mashal?
Rashi: This is Hashem's way ? the fathers sin, and the children are stricken. Malchei Yisrael sinned for many years before they were exiled. Also we need not worry to be stricken for our sin.
Radak: Our fathers sinned, and they are not. We suffer due to their sins. The Nevi'im told Yisrael (the 10 tribes, that they are punished due to) the sins of Yaravam, and Yehudah [due to] the sins of Menasheh. They said, if so, the children are stricken for the fathers' sins. The did not understand that due to the sins of Yaravam and Menasheh there was great anger, for they caused those who came after them to sin. Had Yisrael (and Yehudah) not persisted in the sins of Yered (and Menasheh), they would not have been stricken for their sins and Eretz Yisrael would not have been destroyed. This was Hashem's answer to them in the coming Parashah.
Malbim: Moshe made four decrees on Yisrael, and four Nevi'im nullified them. Moshe said "Poked Avon Avos Al Banim" (Shemos 20:5); Yechezkel nullified it ? "Nefesh ha'Choteis Hi Sanus" (4) (Makos 24a). Chazal asked, it says "u'Vanim Lo Yumsu Al Avos Ish b'Chet'o Yumasu" (Devarim 24:16)! They answered, [Moshe discussed] only when the sons continue in the fathers' evil deeds (Brachos 7a). However, something covered in its place is explicit elsewhere. Aseres ha'Dibros explained that this is for idolatry ? "Lo Sishtachaveh Lahem v'Lo Sa'avdem Ki Anochi Hashem Elokecha Kel Kana Poked Avon Avos Al Banim? l'Son'ai" (Shemos 20:5). Moreh ha'Nevuchim says, we find Charon Af, Chemah, Kin'ah and Sin'ah only regarding idolatry. For it, Hashem eradicates the root with the branches. In Yechezkel's days, Anshei Keneses ha'Gedolah nullified the Yetzer ha'Ra for idolatry (Sanhedrin 64a 1 ). Automatically, "Poked Avon Avos Al Banim" was nullified. For other Aveiros, it is only when the sons continue in the fathers' sins. Until Yechezkel, there was a Yetzer ha'Ra for idolatry, so they made the Mashal of fathers eating Boser. I.e. they served Heavenly legions. This is like an unripe fruit; it sets on edge the teeth of one who eats it, and in this case, the children's teeth, due to Menasheh's sins.
The Gemara there learned from "va'Yiz'aku b'Kol Gadol El Hashem" (Nechemyah 9:4), which was after Binyan Bayis Sheni. Pirkei d'R. Eliezer 33 says that Yechezkel was not buried in a Tahor land (Eretz Yisrael). He is not listed with the Chachamim who returned with Ezra. Did he choose to remain in Bavel?! (PF)
