The above sentence was used as an example sentence in a section of "Du's Handbook" that said when 惟 (유), which means "only," is combined with 爲 (위), they mean "then and only then," similar to the modern Chinese 才 (재). However, I would like to suggest a simpler translation:惟助爲有公田“It is only in the system of mutual aid that there is public land.”
First, the character 助 means "help," but here it is an abbreviation of the word 助法 (조법), which literally means "help (助) method (法)." The "help method" was a system used in ancient China as a way to pay taxes. It worked by dividing a section of land into nine parts, similar to the appearance of the character 井 (정), which means "well." The center section was known as the "public field" (公田 -공전), and the eight surrounding sections were private fields owned by eight different families. The eight families worked together, or "helped" each other, to plant and harvest the public land, where the harvest was used as their tax payment. They were supposed to first work the public field before working their individual fields. Here I translated 助法 (조법) as "Corvée Labor System."惟助爲有公田“Only (惟) the Corvée Labor System (助) creates (爲) [and] has (有) public (公) fields (田).”
Second, the explanation that 惟 (유) and 爲 (위) combined to mean "then and only then" does not make sense to me, at least, not in the above sentence. It made more sense to me that the character 惟 (유) be translated alone as "only," and that 爲 (위) be translated as "to make" or "to create." That means I translated this sentence with two verbs, "to create (爲)" and "to have (有)."
The above sentence comes from the following passage in Mencius:
Notice that the passage from the Book of Poetry asks that it rain on the public field before raining on their private fields. I think that was meant to reflect the fact, maybe humorously, that the farmers had to farm the public field before they could farm their individual fields. My Korean dictionary says that one of the meanings of 亦 (역) is 다스리다, which means "to rule" or "to govern," but can also mean "to manage" or "to administer."詩云雨我公田, 遂及我私, 惟助爲有公田, 由此觀之, 雖周亦助也.The Book of Poetry (詩) says (云), “Rain (雨) [on] our (我) public (公) field (田), then (遂) spread to (及) our (我) private [fields] (私).” Only (惟) the Corvée Labor System (助) creates (爲) [and] has (有) public (公) fields (田), [so] from (由) this (此) observation (觀之), even (雖) Zhou [Dynasty] (周) administered (亦) a Corvée Labor System (助也).”