当从句前后都缺少成分时，能不能用that. 不能的话应该用什么，举个例子吧

Question

如题所述

Answer 1

限制性定语从句与非限制性定语从句。

非限制性定语从句和主句的关系不十分密切, 只是对先行词作些附加的说明, 如果去掉, 主句的意思仍然清楚．这种从句和主句之间往往用逗号分开, 译成汉语时, 往往译成两个分句．引导非限制性定语从句的关系词不可省略, 无论指人、指物, 一般不用that, 要用who, whom和which．
【例】
The conference, which was held last week, was a success．
上周召开的会议是成功的．
The man, whom you met in the library, is our new chemistry teacher．你在图书馆遇到的那个人是我们新来的化学老师．

1．限制性定语从句是修饰先行词的不可缺少的成份，主句与从句一般不用逗号分开。引导限制性定语从句的关系代词有that, which, who(m),
whose；关系副词有when, where, why. 如：
I was the only person in my office who was invited.
我是办公室里惟一受到邀请的人。
He has found the hammer (that/which) he was looking for.
我找到了正在寻找的锤子。(关系代词在从句中充当宾语时，一般可省去。)
This is the question about which we have discussed.
这是我们已讨论过的问题。(介词后不用关系代词that。)
The building whose roof is white is a hospital.
那座屋顶是白色的那栋大楼是一家医院。
The reason why he didn't come is that he was ill.
他没来的原因是由于他病了。
B．非限制性定语从句是对一个概念清楚的先行词(有时为整个句子)作补充说明，一般与主句用逗号隔开。引导非限制性定语从句的关系代词有： which, who(m),
whose关系副词有： when, where(不用 that, why)如：
The man is an officer, whose wife is a doctor.
这人是个军官，他妻子是个医生。
Miss Green is our new teacher, whom you met in the library.
格林小姐是我们的新老师，你在图书馆见过她。(关系代词在从句中充当宾语，引导非限制性定语从句的关系词不省略。)
They flew to Nanjing, where they stayed for 3 days.
他们乘飞机去了南京，在那里他们呆了三天。
We'll put the meeting off till next week, when we won't be so busy.
我们将把会议推迟到下周，那时我们就不会那么忙了。
【注意】
①先行词为专有名词时，一般用非限制性定语从句来修饰。如：
Engles, who was a German, could read and write in several foreign languages.恩格斯能用好几种语言阅读和写作，他是个德国人。
②先行词为普通名词，用限制性定语从句与非限制性定语从句，在意义上有区别。如：His brother who is 18 years old is a PLA man.
他那个18岁的哥哥是个解放军战士。(可能不止一个哥哥。)
His brother, who is 18 years old, is a PLA man.
他的哥哥是解放军战士，现年18岁。(可能只有一个哥哥。)
③先行词如果是修饰整个句子时，用which/as引导非限定性定语从句。which引导的非限制性定语从句不能放在句首。当非限制性定语从句谓语是be
expected, be said, be known, be reported, be announced 等时，则多用as引导非限制性定语从句。
As you all know, he is a doctor. (或 He is a doctor, which/ as you all know.)你们都知道，他是个医生。
Light travels faster than sound, as is known to all.
光传播得比声音快，这一点大伙都知道。