过去分词➕介词
（1）过去分词短语发生的动作在谓语动词之前
（2）短语中的过去分词与逻辑主语之间是被动关系
如：
I saw the ball thrown into the garden.
我看见球被扔进了花园。
过去分词短语可以表达下列四种意思：
㈠方法或活动方式
如：
He walked up and down， lost in thought.
I sat before the desk until after mid-night， absorbed in writing.
Surrounded by a host of fans， the film star left the airport excitedly.
㈡原因，如：
Greatly disappointed， some staff decided to leave the place.
Taken by surprise， the enemy surrendered.
㈢时间，如：
Born and bred in a turbulent age， the older generation of people experienced all sorts of hardships.
Thrown to the floor， the boy regained his footing a few minutes later.
㈣条件
如：
Given more time， the slow learners would have done better.
Criticised by someone else， Tony would not have flared up like that.
除了直接修饰动词之外，过去分词或短语也可以和连词合组成短语，表示下列意思：
⒈由 when， whenever， while， until 等连词引导，表示“ 时间”
如：
When asked about his previous job， Bill said he had been a motor mechanic.
● Susan seldom speaks in class until spoken to.
⒉由 where， wherever 连词引导，表示“地点”，如：
Mosquitoes should be completely exterminated where found.
Retirees in good health should be invited to return to work wherever needed.
⒊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.
