在夸克大全 18.35 Extraposition of aclausal object中写道：
When the object is an -ingclause in SVOC and SVOA clause types, it can undergo extraposition; when it isa to-infinitive clause or a that-clause, it must do so:
You must find it excitingworking here. [1]
You must find working hereexciting. [1a]
I made settling the mattermy prime objective. [2]
* I made it my primeobjective settling the matter. [2a]
这其中，[1]和[1a]是同样的意思，那样为何[2]和[2a]不是，并且[2a]是不对的句子呢？
夸克只不过指出2a是不对的，并没讲解为何是不对的。
《剑桥英语语法》在外置结构的语法制约原因中指出，当主语和表语为确认关系时，外置结构是不允许的。即表语为确认性表语时，不可以用外置结构。比如：
How she escaped is the question we ought to be addressing.
= The question we ought to be addressing is how she escaped.
这个句子的表语为确认性表语，主语和表语可以互换功能。如此的主语是不可以使用外置结构的。下句为错误：
Itis the question we ought to be addressing how she escaped. 错误。
同理，宾语和宾补若是确认性关系，也是不可以使用外置结构的。
I made settling the matter my prime objective.
Setting the matter is my prime objective.
My prime objective is setting the matter.
A=B, B=A, 主语和表语是确认性关系。当宾补与宾语是确认性关系时，宾语不能外置。所以：
I made it my prime objecitve setting the matter. 错误（确认性宾补，宾语不可外置）
You must find working here exciting.
Working here is exciting. exciting为归属性表语
但不可以说：Exciting is working here. (错误）
所以，当宾补为归属性宾补时，作宾语的动名词短语可以外置，也可以不外置。
You must find it exciting working here. 正确（归属性宾补，动名词作宾语可以外置）。
