一事无成 without a single success; achieve [accomplish] nothing; all to no purpose; be [get] nowhere; find oneself a complete failure in life; nothing accomplished; never accomplish anything; without achieving any result
一心不可二用 no man can do two things at once; no man is born wise or learned; one can not be in two places at once
一心不能二用 one can [should] not run after two horses [hares] at the same time
一心 1.(专心) wholeheartedly; heart and soul 一心想[要] set one's mind on; be obsessed with the idea of; be bent on; 一心为革命 wholehearted devotion to the revolution; complete dedication to the revolution; 一心专为人民谋福利 work for the people's interests heart and soul2.(齐心) of one mind; at one 万众一心。 millions of people are all of one mind
一事 [方言] be related (organizationally or professionally); belong to the same organization 我们都是一事。 we are all together