As water is to fish, so air is to man.

as we sweep our rooms, so we should sweep backward ideas from our minds.





As water is to fish, so air is to man.

so在此為代詞，指代前文剛說過的，即指代as從句的表語（as為從句的表語）。

water和air分別是主句和從句的主語，as和so分別是從句和主句的表語，to fish和to man分別是從句和主句的狀語。

比較下面的句子：

Water is necessary/indispensable to fish. Air is necessary/indispensable to man.

二個句子的表語相同，為防止重復，用so替代表語形容詞：

Water is necessary/indispensable to fish. Air isso to man.

so前置：

Water is necessary/indispensable to fish. So air isto man.

so前置可以倒裝：

Water is necessary/indispensable to fish. So is air isto man.

合并二個句子為主從復合句，因為二個句子的表語相同，故用比較分句表示相同，as的用法使得比較分句表語形成空位gap:

As water is to fish, so air is to man.

As water is to fish, so is air to man.



as we sweep our rooms, so we should sweep backward ideas from our minds.

as分句為方法狀語從句，so為連接性副詞，在主句作連接性狀語（語義上而不是語法上連接二個句子sentence或分句clause），表示“也”的意思。