they would not have been prosecuted if they had not lied to police.
虚拟语气
从句 if they had not lied to police是对过去事实的虚拟,用过去完成时代替一般过去时.
译为：如果他们没有对警察撒谎
主句 they would not have been prosecuted 用would+have+done形式
译为：他们就不会被起诉了