plain-clothes policeman

• 便衣警察

plain-clothes policeman的用法和样例：

例句

1. A policeman in plain clothes shadowed the criminal all day.
   有个便衣警察整天秘密监视那个罪犯。
2. Suddenly, many plain policemen emerged and those robbers started to flee.
   突然间很多便衣警察出现了，这些强盗开始逃跑了。