“淤”、“于”、“乎”是上古常用的三个介词,它们的主要语法功能在于引进与动作行为有关的处所、原因、对象、方式,表示比较、被动等用法。由于它们在用法上十分相似,对这三个词的关系历来存在着不同看法。汉魏学者认为这三个字是同义词,如《说文》: Their main grammatical functions are the introduction of premises, reasons, objects, ways, representations, passives, etc., which are related to the actions of the action usage. Because of their similarity in usage, there has always been a difference between the three terms. Han and Wei scholars think these three words are synonymous, such as “Shuo Wen”: