When exactly, will they ask what they will eat in the seventh year?
Ramban: They will pose their question in the seventh year. 1
Seforno: They will pose it in the sixth year. 2
If they were not permitted to plant, how can they say "ve'Lo Ne'esof es Tevu'aseinu"?
Rashi: it refers to the wine, fruit of the trees and the Sefichim.
If Yisrael enjoy a triple-harvest in the sixth year, what exactly is their problem?
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 i the seventh year, what will they eat in the following year, which is Yovel! 3
Seforno: Anticipating the people's lack of faith 4 in the qualitative powers of the fruit, 5 Hashem reassures them that, they will also enjoy a bountiful quantitative harvest in the sixth year, which will suffice for the sixth, seventh and eighth years. 6
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. It is not clear however, how the Pasuk can then state that they plant in the eighth year, which is the Yovel?
Which are not initially discernable.
Seforno (on Pasuk 21): So that they will have the satisfaction of seeing the abundance of food with their on eyes.