The above sentence was used as an example of 以 (이) being used to mean "in," "on," or "at." The problem I have with the translation is not that 以 is translated as "at," but that the sentence is so paraphrased that a beginning student would have trouble understanding how the translation was achieved. Therefore, I would like to suggest the following translation of the sentence, which includes the sentence that preceded it.斧斤以時入山林, 材木不可勝用也“If axes [are only allowed] into the mountain forests at the right time, there will be more timber than can be used.”
When 數 means "number," it is pronounced "수"; when it means "frequently," it is pronounce "삭"; but when it means "close" or "dense," it is pronounced "촉." 罟 (고) means "net," so 數罟 (촉고) means "a close-meshed net." 勝 (승) means "to win," but my Korean dictionary says it can also mean 모두, which means "all." 時 (시) means "time," but it can also mean "season," so 以時 (이시) literally means "with (以) the season (時)." Since "with the season" is an adverbial phrase and in Chinese an adverb precedes the verb it modifies, then 以時 precedes the verb 入 (입).數罟不入洿池, 魚鼈不可勝食也. 斧斤以時入山林, 材木不可勝用也.“[If] close-meshed nets (數罟) do not (不) enter (入) pools (洿) [and] ponds (池), the fish (魚) [and] turtles (鼈) cannot (不可) all (勝) be eaten (食也). [If] axes (斧斤), with (以) the season (時), enter (入) the mountain (山) forests (林), the timber (材木) cannot (不可) all (勝) be used (用也).”
The passage comes from "Mencius," who was suggesting that if close-mesh nets were prohibited and woodcutting restricted to only a certain season, then those resources could be sustained. Larger-meshed nets would allow smaller fish and turtles to escape and grow into bigger fish and turtles, and restricting woodcutting to only a certain season would allow smaller trees to grow into bigger trees.