不定式做目的状语,和主句不能用逗号隔开吗

当然可以,不过此时它一般放在句首,可以和in order to换用.如：To save class time,our teacher has us students do half of the exercises in class and complete the other half for our homework.= In order to save class time,our teacher has us students do half of the exercises in class and complete the other half for our homework.为了节约课堂时间,我们的老师要求我们学生在课堂上做一半练习,剩下的一半在课后作为家庭作业完成.如果放在句中,则做目的状语的不定式最好不要用逗号隔开,以免被误认为是结果状语.