为什么as后面加过去分词

as后面接过去分词是因为，省略了助于和动词，一般是表被动的时候，会用到

如：

As is known to all， China is a developing country. 

as 为关系代词，做主语，在此代指后面整个句子。

As we all know， China is a developing country.

这句话中，as为关系代词，作宾语，指代整个句子

as + 过去分词  省略结构：

用于此句式的动词，一般是表示“计划，想象，预料，叙述，描述，提及，说及，显示，指出”之类的动词，多用于议论文体。

在过去分词之后还可以加一些其他成分

如：as shown at page two， as shown by this picture。

常用的有：

as planned 正如计划的那样

as scheduled 正如安排的那样

as expected 正如预料的那样

as imagined 正如想象的那样

as arranged 正如计划的那样

as seen from above 正如从上述看的那样

as told before 正如前述

as shown above 正如上面显示的那样

as said 正如说过的（那样）

as mentioned 正如提到的（那样）

as described 正如描述的（那样）

as discussed 正如讨论过的（那样）

as noted 正如叙述到的（那样）

as stated 正如说指出的（那样）

as announced 正如宣布的（那样）

as pointed out 正如已经指出的（那样）

as indicated 如前所示（那样）

as hinted 正如已经暗示的（那样）

as told 正如已经告知的（那样）

as already shown 如已显示（那样）

as emphasized 正如已经强调的（那样）

as observed 正如已观察到的（那样）

as explained 正如已经解释过的（那样）

as suggested 正如已经提出的（那样）