SCARLETT O’HARA was not beautiful, but men seldom realized it when(they were 省略了) caught by her charmas the Tarleton twins were. 

参考译文：思嘉·奥哈拉长得并不漂亮，但是当男人们像塔尔顿家那对孪生兄弟为她的魅力所迷住时，就不会这样想了。

(备注：when were by her charms the Tarleton twins 作时间状语作用。）

以上为保留连词+分词作状语结构

Unless changed, this law will make life difficult for farmers. 

除非加以修改，否则这条法律将给农民的生活造成困难。
When asked why he came here, the boy kept silent.

当那个男孩被问到为何来这里时，他沉默不语。

SCARLETT O’HARA was not beautiful, but men seldom realized it when caught by her charms the Tarleton twins were.

下划线的这段句子，应该是when the Tarleton twins were caught by her charms吧。这是不是属于倒装句，是什么样的语法结构呢？它有什么用处呢？

【答】when caught by her charms the Tarleton twins were. = when they were caught by her charms the Tarleton twins were.

分词（包括现在分词、过去分词）作状语时，为了明确它属于何种状语，可以在分词之前加上必要的连词。例如：以下连词 when, while, until, once, though, although, as long as, so long as, unless, if, as if, as though, even if, even though 都可以添加。

实际上，也可以理解这样理解，为了表达简练，在从句中省略了“主语 + be”。

例如：

1. He will come if invited.= He will come if he is invited. 若受到邀请，他就来。

2. Unless asked/invited，I won't attend the get-together. 除非受到邀请，否则我不会参加聚会。

3. Don't talk while having dinner. 吃饭时不要说话。

4. When crossing the street，do be careful. 过马路时一定要小心。

5. Once losing this chance，you can't easily find it. 一旦失去这次机会你就很难找回。

6. As long as going into the reading room，you should keep quiet. 只要进入阅览室，你就应保持安静。