A is to B as/what C is to D. 

主句：A is what C is(to B为主句状语，to D为从句状语）

在以上结构中：what从句是名词性从句，what既引导表语从句，同时在从句中做表语。（名词性表语)。to B和to D分别是主句和从句的状语。

http://ask.yygrammar.com/q-23737.html

A to B is what C to D”中，A.B均为本体，C.D均为喻体，“what”作为连词使用，意思是“恰如、好像、相当于”， what前后为物与物的比较，如果空格前后为数与数的比较，则要用as连接，

Four is to twelve as one is to three．4比12等于1比3。

Food is to the body what reading is to the knowledge．

食物对于身体正如读书对于知识。

Oxygen is to the human beings what water is to the fish．

氧气对于人类就像水对于鱼。

Reading is to the mind what food is to the body.

读书之于心灵，犹如食物之于身体。

Three is to four as six is to eight. 三对四之比等于六对八之比。

Air is to man as water is to fish. 空气之于人犹如水之于鱼。

Water is to fish, as air is to men. 水之于鱼，犹如空气之于人。

这样用的 as 也可以换成 what。如：

Manure is to crops what food is to man. 

肥料对于农作物，就如同食物对人一样。

Engines are to machines what hearts are to animals. 

发动机对机器之关系犹如心脏对动物之关系。

关于“A is to B as C is to D.”的句式，第2种解答

Leaves are to the plant what lungs are to the animal.

叶之于植物犹如肺之于动物。

Regulation is to a school what the law is to a nation.

校规之于学校，犹如法律之于国家。

这个句型与下列句型相似，试比较：

① A is to B what X is to Y 

② A is to B as X is to Y 

③ As X is to Y, so is A to B 

④ What X is to Y, that is A to B

在①、②句型中，what和as都用作连接词，其后是从句，主句在前。①句型可译为“A之于B等于X之于Y”，其中what X is to Y是名词从句，是动词is的补语，动词is可以省去。②句型可译为“A之于B犹如X之于Y”。③、④句型与①、②句型相反，主句在后，从句在前。③句型可译为“正如X之于Y，A对于B也表示同一关系”。在④句型中，作为补语的名词从句放在句首，用that作同格词，这个that可省去。注意③、④句型的后半句，主语和动词的位置转换了。

其他类似的例子：

Air is to us as water is to fish. 

空气对于我们，犹如水对于鱼一样重要。

Reading is to the mind what exercise is to the body.

读书对于思想，好比运动对于身体一样重要。

What the voyage of Columbus was to the world in the 15th century, the journey of the satellite, Lunik Ⅲ, might well be to the twentieth. 

哥伦布的航行对于15世纪人们的意义，犹如3号月球卫星航行对于20世纪人们的意义一样。

As the house is to the man so is the nest to the bird and the cave to the animal

= The house is to the man what the nest is to the bird and cave to the animal.

= What the nest is to the bird and the cave to the animal so is the house to the man.

室之于人，犹巢之于鸟，穴之于兽。

Five is to seven what ten is to fourteen. 

五与七之比等于十与十四之比。