1.A quarter of the students (have left)their classroom.这dao题

1.A quarter of the students (have left)their classroom.这dao题大概不是现在完成时吧、
2.the factory has been (shut down)
3.the new train without wheels hasn't been (in use)it yet
4.old people must (be spoken to politely)
5.l will finish the work (next day)
he said he would finish the job (tne next day)为什么这两个一个加the,一个不加?
尽量写详细些,
括号里的是正确答案、 第三题没有it、
我感觉第一题如果是现在完成时，答案应该是have been away，因为要用延续性动词。