请问,目的状语从句的虚拟语气的用法是什么?时态有什么规定吗?

目的状语从句的虚拟可以分为两类。一、由 so that,in order that,引导的目的状语从句中,需要在从句的谓语语动词前加情态动词 can,may,could,might,前两个只用于现在时态,后两个既可以表示委婉的现在也可以表示过去,如：* He works hard so that he can pass the exam。他努力学习,以便能够通过考试。* He worked hard so that he could pass the exam。（当时）他刻苦地学习,以便能够通过考试。* In order that he may pass the exam,he works hard。为了能够通过考试,他努力学习。* He worked hard in order that he might pass the exam。为了能够通过考试,他当时努力地学习。* He will come on time so that he could know everything from beginning。(委婉语气) 二、由 lest,for fear that 引导的目的状语从句,表示“以防、唯恐、免得”,之意,其谓语动词要用 should do,如：* She tiptoed lest the guard should hear her。她踮起脚尖走路,以免守卫发现她。* He ran away for fear that he should be seen。唯恐被人发现,他逃离了。