We won't start the work until all the preparations has been made.
1.在时间状语从句中,表示以后已完成的被动动作动作，用目前完成时的被动语态代替以后完成年代的被动语态。这里是说不管主句是什么时态，从句都要用目前完成时的被动语态吗？
2.preparations是复数，为何用has？
We won't start thework until all the preparations have been made.
【翻译】直到所有些筹备都做好了，大家才开始工作。
has 一定是不对的，改为 have。
你的问题，从本质上看就是：目前完成时用在“时间状语从句”或者“条件状语从句”中，表示以后业已完成的动作或状况，代替以后完成时。
目前完成时用于时间状语从句举例：
Stay in the houseuntil the danger has passed. 躲在家，直到危险过后再出来。
I'm very busy justnow; wait until things have eased up a little. 我目前很忙，等事情缓一缓再说。
Don't drink thewater until it has been cooled. 直到水冷了之后再喝。
We can't move inuntil the electricity has been laid on. 大家得等电源接通后才能搬进来。
I'll go to mygranny's as soon as I have finished my homework. 我一做完作业就去奶奶家。
As soon as he hasregained his memory, the problem will be solved. 只须他一恢复记忆，问题就能解决了。
When you haveformed a plan, you must carry it out. 计划订出之后，你需要贯彻实行。
The hen cackleswhen she has laid an egg. 母鸡下蛋就咯咯地叫。
When you havewashed the car, wipe it down well . 你把汽车冲洗过后，要好好使布揩拭一下。
Don't forget toswitch off when you have finished. 做完之后别忘了关掉。
Put the book backwhen you have done with it. 你读完这本书时把它放回原处。