有一读者问： 有这样两个句子：
(1)Only once did the gentleman mention his view of life.
(2)Before George stood the policeman who had arrested him before for drug-taking.
为什么上述句（1）加did，而句（2）却不加？
倒装一般有两种：句子结构的需要和强调部分词语的需要。上述句（1）的倒装乃属于强调部分词语的需要，即强调状语only once。强调状语时，状语须置于句首，后接倒装结构，即be, have，助动词（包括情态动词）等须置于主语之前，再如：
(3)Not once did he talk to me.
(4)Only in this way can we learn English.
上述句（2）的倒装则属于句子结构的需要，具体而言，是由于主语部分（the policeman who...）较长的缘故。在这种倒装结构中，主语之前可用完全动词，再如：
(5)Then began a bitter war between the two countries.
(6)Now follows a description of how nature appeared to man.
(7)In a cage about five metres high in the Kunming Zoo live two black monkeys with long hair, beautiful faces and pink lips that make them look as if they have been using lipstick.
