从句判断（His height is double what it was...）

His height is double what it was three years ago.

请问,句式上不太了解这是什么从句?

His height is double what it was three years ago.

what it was three years ago 为名词性关系分句，what在这里等于the heigh that。所以，这个句子等于：

His height is double the height that itwas three years ago.

double为倍数词，倍数词+the/物主代词+名词=是...的n倍。比如：

His salary is three time your salary. 他的薪资是你的三倍。

所以，His height is double what it was three years ago. 意为：他的身高是三年前身高的二倍。

这个句子中的He只能是个新生婴儿，三岁时的身高是刚出生时身长的二倍。此外，这个句子表示的逻辑很难想象。