请看这道题，这个until 后要接过去分词吗?
He is a man of few words, and seldom speaks until _________ to.
A. spoken B. speaking C. speak D. be spoken
【分析】此题容易误B，认为until 是介词，后接动词时用动名词形式。其实，此题应选A，until spoken to 为 until he is spoken to 之省略。句意为“他是个沉默寡言的人，别人不同他说话，他很少同别人说话”。按英语习惯，一些表示时间、地点、条件、方式、 让步等的状语从句，若其主语与主句主语一致，且从句谓语包含动词be, 那么可将从句的主语和动词be省略：
You must study hard while (you are) young, or you will regret when (you are) old. 趁年轻时要努力学习，不然到老了你会后悔的。
I won't go unless (I am) invited. 我不会去, 除非请我。
Look out for cars when (you are) crossing the streets. 过马路时要注意汽车。
While (I was) waiting I was reading some old magazines. 等的时候我在看一些旧杂志。
He worked very hard though (he was) still rather poor in health. 尽管身体还不好，但他仍努力工作。
He will work hard wherever (he is) sent by the Party. 无论党把他派往哪里,他都会努力工作。
 
请再看两道试题：
 (1) If carefully _________, the experiment will be successful.
A. do B. does C. done D. doing
答案选C，可视为 if it is carefully done 之省略。
 (2) The research is so designed that once _________ nothing can be done to change it.
A. begins B. having begun C. beginning D. begun
答案选D，可视为 once it is begun 之省略。