They are unarmed plain-clothes men.

他们是不带武器的便衣警察。

That's certainly a plain-clothes policeman.

那肯定是个便衣警察。

Plain-clothes policemen have been planted at all the exits.

在各个出口都安置了便衣警察。

Plain-clothes policemen had been planted at all the exits.

所有的出口处都布置了便衣警察。

Plain - clothes policemen have been planted at all the exits.

在各个出口都安置了便衣警察.

Only after she had been writing her exam paper for two hours did plain-clothes police arrive and wait outside the exam hall.

她在答卷仅两个小时后，便衣警察就到外头的考试大厅候着了。