到了C1水平，规范的英语和流利的英语之间的差别，往往就体现在密度上：在不另起一句的前提下，一个名词上能挂多少信息量。
限定性从句和非限定性从句
限定性从句——说明的是「究竟是哪一个」。不用逗号。
The report that we published in June has been withdrawn.
去掉它，就不清楚说的是哪份报告。这部分信息是必需的。
非限定性从句——补充一个细节。必须加逗号。
The report, which we published in June, has been withdrawn.
报告只有一份，从句只是附带说明了一个事实。逗号会改变句意，这是C1水平最常见的标点错误。
My brother who lives in Berlin is a doctor.（说话人有好几个兄弟）
My brother, who lives in Berlin, is a doctor.（说话人只有一个兄弟）
由此得出两条规则：非限定性从句里不能用that，非限定性从句也不能省略关系代词。
什么时候可以省略关系代词
在限定性从句里，如果关系代词在从句中充当宾语，就可以省略：
The book (that) I ordered hasn't arrived.（宾语——可以省略）✅
The book that arrived was damaged.（主语——不能省略）✅
一个快速判断法：如果紧跟在后面的是一个名词或代词，说明它是宾语，可以省略。
介词
The colleague who I spoke to.（非正式）
The colleague to whom I spoke.（正式——介词前置，必须用whom）
绝不能说to who。如果介词被提到前面，就必须用whom。
Whose、where、when
- the company whose licence was revoked（表示所属关系——用于物同样成立）
- the year when the policy changed = in which
- the point where the argument fails = at which
在限定性从句里，where和when可以像that一样被省略：the day (when) we met · the place (where) we stayed。
指代整句话的which
非限定性的which可以不指代某一个名词，而是指代整个意思——不少C1水平的评论性表达都建立在这个结构上：
- He arrived an hour late, which didn't help.
- They cancelled without warning, which is exactly what I'd expected.
只有which才能这么用。❌ He was late, what annoyed everyone.
数量词 + of whom / of which
正式书面语用这个结构，把关于同一个群体的两个事实合并起来，而不是另起一句：
- We interviewed forty candidates, most of whom had no experience.
- The company runs six sites, two of which are due to close.
- She made several points, none of which were addressed.
这里只能用whom和which——绝不能用who和that。
缩略的分词短语
带有进行时或被动形式的限定性从句，会省去关系代词和助动词：
The people who are waiting outside → the people waiting outside
The measures that were introduced last year → the measures introduced last year
正是这一点，让正式英语显得如此紧凑：
Applicants submitting incomplete forms will be contacted.
Funds allocated to the scheme remain unspent.
放在句首的分词
同样的形式也可以放在句首，代替because、after或while：
| 形式 | 代替 | 例句 |
|---|---|---|
| -ing | while / because | Working from home, I miss the office. |
| Having + 过去分词 | after | Having finished the report, she left. |
| 被动分词 | because / when | Written in 1890, the book still sells. |
| Being + 分词 | because | Being new, he asked a lot of questions. |
陷阱： 分词必须和紧挨着它的那个名词对应。
❌ Having reviewed the data, the conclusion was obvious.（结论本身并没有研究任何数据） ✅ Having reviewed the data, we found the conclusion obvious.
为什么这是C1水平的一道门槛——B2水平的写作者会用三个短句表达的内容，C1水平的写作者只用一句话就能说清楚。信息量是一样的，区别在于读者是否需要自己把信息重新拼起来。每挂在一个名词上的从句，都省去了一句本可能另起的句子。
把同样的内容用两种密度对比一下：
| 三句话 | 一句话 |
|---|---|
| We published a report in June. It recommended six changes. Two of them have been adopted. | The report we published in June made six recommendations, two of which have been adopted. |
自测
下面的测验涉及逗号的使用、可省略的关系代词、前置的介词，以及缩略结构。