One knows it at a glance.
一望而知。
One flower doesn't make a spring.
一花獨放不是春，百花齊放春滿園。
One bite won't make a fat man.
一口喫不成一個胖子。
One false step causes eternal regret.
一失足成千古恨。
One can't draw a circle with one hand and a square with the other.
一心不能二用。
One barking dog sets all the streets a barking.
一犬吠影，百犬吠聲。