前者出现的情况如will be done,must be done 等,前有情态动词,其它的be要用具体的,如is/are/was/were done
如The work will be finished.工作将要完成.
The plan must be carried out.计划务必执行
The clothes were washed.衣服洗了.
后者是不定时的被动式,
如That remains to be seen.那件事不得而知.
My car needs to be repaired.我的车要修理