When exactly, will they ask what they will eat in the seventh year?
Ramban, Da'as Zekenim, Hadar Zekenim, Riva, Rosh: Transpose the verse to say, if you will ask in the seventh year 'what will we eat [next year]?'. 1 Normally, people eat what grew the previous year. 2
Seforno implies that they will pose it before the sixth year (if they do not trust that a normal year's harvest will satiate them for additional years). 3
If they were not permitted to plant, how can they say "v'Lo Ne'esof es Tevu'aseinu"?
Rashi: it refers to the wine, fruit of the trees
According to Targum Yonasan, it refers to Sefichim, exclusively.
If Yisrael enjoy a triple-harvest in the sixth year, what are they worried about?
Rashi: It is in anticipation of the people's concern as to what they will eat in the Shemitah that He reassures them that the sixth year will yield a triple harvest to last until the eighth year. 1
Ramban: The Pasuk is speaking about the seventh Shemitah of the cycle, and they are concerned 2 that, if they eat what they have in the seventh year, what will they eat in the following year, which is Yovel! 3
R. Uziel Milevski, according to Seforno: Ideally, the land produces the usual amount, and due to the qualitative Brachah, it lasts for two or three years. This saves time of harvesting, processing food and eating. If they are worried, instead they get a quantitative Brachah. 4
Ramban: To which Hashem replied that, under these circumstances, He will bless them in the sixth year with sufficient for three years - Shemitah, Yovel and the year after Yovel. How can the coming Pasuk state that they plant in the eighth year, which is the Yovel? Perhaps every Shemitah there is concern for the year after Shemitah; the Ramban explains that when Yovel is after Shemitah, the land must produce for three years. (PF)
Seforno (21): And they will have the satisfaction of seeing that there is enough food.
From where do they reap for the Omer and Shtei ha'Lechem in Yovel? Everything was finished!
Moshav Zekenim citing R. Yehudah ha'Chasid: There was a field special for this; people did not touch it. Surely [Sefichim of grain] fell in Shemitah, and sprouted in Yovel. 1
According to R. Akiva, who forbids Sefichim mid'Oraisa, how can we answer? If it is forbidden to people, it is forbidden to the Mizbe'ach! (PF)