解析：is walking 是现在进行时,是句子的谓语,这两个词的顺序不调换,所谓倒装是指句子的主语和谓语的顺序调换,即本句的正常顺序是
Look over,a policeman is walking around the corner.
倒装句就为你写的句子：Look over,around the corner is walking a policeman.
翻译：仔细看,一个警察正围着角落转.
有不明白的请继续追问,