as different as chalk and cheese 截然不同，本质上完全不同

【例句】
The two brothers resembled each other physically, but were as different in their natures as chalk and cheese.
这兄弟二人外貌相像，但性格截然不同。

The two places, he insists, are as different as chalk and cheese.
他坚持说那两个地方有天壤之别。