It was a simple stone—you may still see it there today and on it was just one letter；

翻译：这是一块很普通的石头——今天你可能还会在那儿看到它，在那上面只有一个字母。

请问：on it was just one letter 是一个完整的句子吗？on it 难道是主语吗？

on it介词短语作表语倒装结构

A:介词短语作表语倒装结构：

(倒装结构：介词短语+系动词+主语）

In the box was a cat.

箱子里是一只猫。

In the box were some cats.

箱子里是一些猫。

At the next table was a pretty girl waiting for someone.

隔壁桌上坐着一个等人的漂亮姑娘。

Among these people was his friend Jim.

他的朋友吉姆就在这些人当中。

At the front of the book is a table of contents, giving details of what is in the book.    

书的前部有目录，详列了书中的内容。

B:介词短语作状语倒装结构：

(倒装结构：介词短语+谓语+主语）

On the door hung a big sign.

 门上挂着一个大招牌。

Over the wall came a shower of stones.

从墙上飞来一阵石块。

Round the comer came Mrs Porter.

波特夫人在拐角上出现了。

On the top of the hill stood an old castle.

山丘上有一座古堡。

Under the road run pipes for gas and water.

在马路下面铺着煤气管道和自来水管。

Before them lay miles of undulating moorland.

他们前面是一片高低起伏的荒原。

In the distance could be seen the purple mountains.

远处可以见到紫色的山。

At the top of the hill stood the tiny chapel.

那座小教堂矗立在山顶上。

In the fields of poppies lay the dying soldiers.

罂粟地里躺着奄奄一息的士兵们。

In the distance could be seen the purple mountains.

远处可以见到紫色的山。

By the window sat a young man with a magazine in his hand.

窗户边坐着一个年轻人，手里拿着一本杂志。

(特别备注： 主语如是代词则不能倒装）

At the top of the hill it stood out against the sky.

它背衬青天矗立在山顶上。