● Retirees in good health should be invited to return to work wherever needed.

（3）由 if, unless 引导，表示"条件"，如：

● If kept for too long, some medicines will lose their effectiveness.

● We have made a point of not attacking unless attacked.

（4）由 though, although, even though 连词引导，表示"让步 "，如：

● Though warned of the danger, they still went mountaineering.

● Even though defeated for a second time, our team did not give up hope for the ultimate victory.

此外，过去分词短语还可以和介词"with"或"without"连用，具副词作用。如：

● With the water pipe choked, there wasn't any more water for use.

● Without anything left in the kitchen, the Wangs decided to eat out.

最后，过去分词短语，在适当情况下，可以有自己的主语而变成独立结构（the absolute construction），如：

　● The old man listened, his head inclined to one side

4. Studying the past participle as the attribute

过去分词作定语，在语态上，表被动；在时间上，表示动作已经发生或完成，与它所修饰的名词有逻辑上的动宾关系。过去分词作定语时，所修饰的名词在逻辑上相当于被动句中的主语，过去分词相当于谓语。

过去分词作定语表示动作在谓语动作之前发生，已经完成并具有被动意义。有时也不表示时间性。作定语的过去分词一般由及物动词变来，因为只有及物动词才有被动意义。例如： He is a teacher loved by his students. 他是个很受学生爱戴的老师。

也有用不及物动词的过去分词作定语的情况，一般作前置定语，它不表示被动意义，只表示主动意义，强调动作完成。不能像及物动词的过去分词那样放在名词后面作定语。例如： fallen leaves 落叶, retired workers 退休工人, the risen sun 升起的太阳

注意下面过去分词作定语的几种情况：

A. 单个的过去分词作定语一般放在被修饰的名词之前。例如：

We needed much more qualified workers. 我们需要更多的合格的工人。

My friend is a returned student. 我的朋友是个归国的留学生。

单个分词也可以作后置定语，用以强调动作。例如：

They decided to change the material used. 他们决定更换使用的材料。