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问题描述
Before being used,the machine must be checked. 这里的before是介词还是连词？可以省略being吗？
老师答疑
陈老师：
Before being used,the machine must be checked.
这里的before是介词还是连词？可以省略being吗？
【1】before being used, the machine must be checked
=Before the macine is used, it must be checked.
以上属于介词用法
句子语法结构分析如下：
时间状语：before being used(介词短语充当)（介词+动名词被动语态结构）
主语：the machine（这台机器）
谓语：must be checked（必须被检查）
中文意思：这台机器在使用前必须被检查.
（介词使用before后可接名词/代词/动名词）
The man standing before the blackboard is our English teacher.
站在黑板前面的那个人是我们的英语老师。
We got up before sunrise.
我们在太阳升起之前起床。
Before crossing the road, you should have a look at both ways.
过马路之前,你应该看看路的两边。
Quality comes before quantity.
品质优于数量。
We must stop and rest before going any farther.
我们必须停下来休息一下再向前去。
You must pay attention to this problem before every thing.
你应该首先注意这个问题。
The criminal case comes before the court.
那桩刑事案件已送法院审理。
Great sums of money were placed before the scientists by the government.
政府为科学家们提供了大笔的经费
What do you earn before tax?
在未缴税前你的收入是多少?
He would die before betraying his country.
他宁愿一死,也不肯叛国。
【2】before the macine is used, the machine must be checked
=Before it is used ,the machine must be checked
=Before the machine is used, it must be checked.
如果省略being 句子结构发生一些改变，就是before 当连词使用 引导的（时间状语从句作时间状语，其它方面保持不变，请看下面
(before 作为连词用法+完整从句结构）
Be a pupil before you become a teacher.
先做学生,再做老师。
He will come to see you before he leaves Beijing.
他离开北京之前会来看你的。
It will be some time before we know the full results.
还要过一些时间我们才能知道全部结果。
It will be eight years before we meet.
八年以后我们才能相见。
His father died before he was born.
他父亲在他出世以前就去世了。
He had left for the United States just before the letter arrived.
他恰好在信到之前去美国了。
She died before she reached the hospital.
她还没有到医院就死了。
I had barely come in before the telephone rang.
我刚进来,电话就响了。
Barely had she sat down before the play began.
她刚坐下,戏就开始了。
We had scarcely left the house,before it began to rain.
我们刚刚离家，天就下起雨来。
I will die before I'd surrender.
我宁愿死也不投降。
He'd sit alone in the dark before he'd watch TV.
他宁愿一个人在黑暗里坐着也不看电视。
We must stop and rest before going any farther.
我们必须停下来休息一下再向前走。
谢谢你的回答。我想问下，把before看作连词能省略being。我看到很多状语从句可以省略成连词+过去分词（短语），是没有being的
继续回答如下：
(1)由于 before 既可当连词，又可当介词使用，因此当连词使用后面要接完整句子，从句一般不可以采用省略形式，（但是当介词用，可以该为当连词使用从句.）
(2)while, when 只能当连词使用，因此，引导从句，可以省略从句“主语+谓语”部分，条件为从句和主句有相同主语，且从句含有be 形式，或者(it is )形式
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