判若鸿沟 as different as night and day
Explanation
比喻界限分明，区别很大。
It is used to describe a clear distinction and a huge gap.
Origin Story
话说秦末楚汉相争之际，项羽和刘邦在鸿沟划界，约定以鸿沟为界，划分天下。鸿沟以西归汉，鸿沟以东归楚。此后，楚汉两军虽然表面上休战，但暗地里却暗流涌动，互相猜忌。项羽最终兵败垓下，自刎而死，而刘邦则建立了汉朝，一统天下。鸿沟虽只是一条小小的河，但却成为了楚汉两军分界线，象征着两军之间的巨大差距，也成为了历史上一段重要的分界线。两军实力对比悬殊，宛如天壤之别。这段历史也告诉我们，实力悬殊，结果往往显而易见。
During the Chu-Han contention at the end of the Qin Dynasty, Xiang Yu and Liu Bang agreed to use the Honggou River as a boundary to divide the country. West of the Honggou River belonged to Han, and east of it to Chu. Although the two armies superficially agreed to a truce, there was distrust and intrigue beneath the surface. Xiang Yu was eventually defeated at Gaixia and committed suicide, while Liu Bang founded the Han Dynasty and unified the country. Although the Honggou River was just a small river, it was the boundary between the armies of Chu and Han and symbolized the great difference between them. This historical event teaches us that a disparity in power often predetermines the outcome.
Usage
形容界限分明，区别很大。多用于比喻意义。
It is used to describe a clear distinction and a huge gap. Often used in a figurative sense.
Examples
-
他俩的观点判若鸿沟，根本无法沟通。
tā liǎ de guāndiǎn pàn ruò hónggōu, gēnběn wúfǎ gōutōng
Their viewpoints are worlds apart; there is no way to communicate.
-
改革开放前后，中国的变化判若鸿沟
gǎigé kāifàng qiányòu, zhōngguó de biànhuà pàn ruò hónggōu
China before and after the reform and opening-up is worlds apart