有一读者问这样一道选择题：
(1)Nine thieves had been seen_________and_________into the police car.
A. catch...throw B. to be caught...to be thrown
C. catching...throwing D. caught...thrown
这道题为什么选B而不选D?
这一道题可填以caught...thrown，也可以填to be caught...thrown（后者较前者正式）。如要分别正式叙述两个动作，亦可用B。B这种结构在语法中叫做双重被动式结构，其中第二个被动式一般皆用不定式的被动式，如：
(2)The books are not allowed to be taken out of the room.
(3)Leaves are not meant to be burnt .
(4)More than 250 tons of oranges are expected to be harvested this year in the country.
(5)A new plan was attempted to be formed .
但也有一些例外，即第二个被动式可不用不定式的被动式，而用过去分词，如：
(6)The streets were ordered cleaned .
(7)Last night the airports at Xi'an and Zhengzhou were reported closed by the rain.
(8)Others were believed still trapped on the free floating ice chunks.
这种可用过去分词的结构的第一个被动式除可用上述及物动词order, report, believe外，还可用want, see, hear, find等及物动词。
