(Ichiyou ochite tenka no aki wo shiru;
“A leaf falls, and one knows autumn”)
Sensing something big coming on from a relatively minor hint. Understanding the future from the very first signs. Like realizing that fall is on its way after seeing the fall of a single leaf. In some, but not all, cases, this may specifically refer to presentiments of some sort of downfall.
We begin with number-noun 一葉 (ichiyou), “one leaf,” particle elided but acting as the subject of the verb 落ちる (ochiru), “to fall.” This appears in conjunctive form and prefaces the action of the following clause. The particle を (wo) marks the noun 秋 (aki), “autumn,” as the object of this clause. Associative particle の (no) allows this noun to be modified by the preceding noun 天下 (tenka), which, if you recall, refers to the (mortal/material) world as a whole. And the verb acting on aki is 知る (shiru), “to know,” which appears in conclusive form.
In keeping with this history, there are many variations on the basic phrase. Some specify that the leaf in question belongs to the 青桐 (aogiri, the Chinese Parasol tree), apparently known to start dropping its leaves relatively early; others condense the saying in various ways, for example, by eliminating the 天下 part or even the final verb.
(Kare wa ichiyou ochite tenka no aki wo shiru koto wo mezashite sasai na genshou ya dekigoto ni yatara to binkan ni natta ga, kisoteki na chishiki no fusoku no tame, saisan-saishi shippai de owatta.)
[“Aiming for the ability to foresee big events from their first signs, he became excessively sensitive to even the most insignificant events and phenomena. But lacking any sort of foundational knowledge, his efforts ended in failure time and time and time again.”]