英语常把大量信息放在名词之后。阅读者必须判断后置信息是在缩小对象集合、补充旁注，还是说明一个抽象名词的内容。
逗号为什么能改变关系从句的含义？that 从句何时修饰对象，何时说明内容？答案来自“先行词集合是否需要被筛选”。
Students who submitted early received feedback.
无逗号时，who... 筛选出提前提交的学生，不承诺所有学生都提前提交。
The students, who submitted early, received feedback.
逗号把学生视为已识别整体，并补充他们都提前提交的信息。
The measure that the researchers developed captures trust.
that 在关系从句中作 developed 的宾语，整个从句限定 measure。
The claim that trust increased remains uncertain.
that 不在内部占空位；它引出 claim 的内容，是内容从句。
The final sample, 312 households from two districts, was smaller than planned.
中间名词短语与 the final sample 同位，重新说明样本是什么。
The conditions under which the model is valid are restrictive.
介词前置后，which 仍是 under 的补语；关系从句限定 conditions。
The report includes three cases, two of which contradict the theory.
补充性关系结构量化前述三例中的两个，不能改成限制性 that。
Policies intended to increase access may unintentionally raise costs.
intended... 是缩减的关系修饰，约等于 that are intended...，限定 policies。
The dataset contains schools that adopted the program voluntarily and schools that were assigned to it by the district. The first group, which had greater freedom of choice, also had stronger administrative support. That difference creates a problem: the schools that chose the program may have differed before implementation. The assumption that the two groups were initially comparable therefore requires independent evidence.
两个限制性关系从句划分学校集合；which... 为已识别的第一组补充信息；最后的 that 从句说明 assumption 的内容。三种结构形式相近，功能不同。
训练语境：学术科研
比较：studies that used surveys 与 the studies, which used surveys。
前者从所有研究中筛选使用问卷的子集；后者假定研究集合已确定，并补充这些研究都用了问卷。逗号改变的是集合解释，不只是停顿。
训练语境：学术科研
判断 that：the evidence that the committee reviewed。
这是关系从句，that 是 reviewed 的宾语，表示委员会审查的那部分证据。若写 the evidence that the policy failed，后者才更像内容从句。
训练语境：学术科研
解释：The participants, many of whom were retired, preferred the simpler form.
many of whom 对已识别参与者作部分量化旁注，说明其中许多人退休。它不用于筛选“哪些参与者”进入主句。
训练语境：学术科研
为何 which 可能回指整个命题：The estimate doubled, which surprised the team.
这里被 surprising 的不是某个名词，而是“估计翻倍”这件事。补充性 which 可以对前面整个命题发表评论。
训练语境：学术科研
分析：the possibility of error 与 the possibility that the measure is biased。
前者用介词短语概括错误这一可能性；后者用完整内容从句明确偏差命题。两者都补充 possibility，但信息精度不同。
训练语境：学术科研
补全：the mechanism through ___ the effect operates。
正式结构用 which：through which。关系词是介词补语；口语也可说 the mechanism which the effect operates through，但不能在介词前用 that。
训练语境：真实生活
比较：My brother who lives in Boston 与 My brother, who lives in Boston。
无逗号可能暗示我有不止一个兄弟，需要用居住地识别；有逗号则把 brother 视为已唯一识别，再补充居住地。
训练语境：真实生活
解释同位语：Maya, our new manager, starts Monday.
our new manager 与 Maya 指同一人，重新命名而非描述另一个对象。删除同位语后主句仍是 Maya starts Monday。
训练语境：真实生活
消歧：I returned the books that were damaged.
限制性从句通常表示只退还损坏的书，不能推出所有书都损坏。若所有相关书都损坏并只是补充说明，应写逗号结构。
训练语境：真实生活
判断：the news that you mentioned 中 that 的功能。
that 在 mentioned 后作宾语，因此是关系从句，意思是“你提到的消息”。the news that the store closed 才是说明消息内容。
The archive, which contains letters written over four decades, has often been treated as evidence of a single political conversion. That interpretation, however, overlooks a simpler possibility: that the writer used different vocabularies for different audiences. The letters that were sent to party officials emphasize loyalty, whereas those addressed to family members describe uncertainty and fatigue. The contrast suggests not hypocrisy, but audience-sensitive self-presentation.
a simpler possibility。which contains... 只是补充说明 archive，不限定是哪一个 archive。that the writer used... 是 possibility 的同位内容。letters that were sent... 中的 that 是限制性定语从句，区分写给官员的信。those addressed to family members 中 those 代替 letters，后面的过去分词短语作后置修饰。高阶语境：学术科研
比较第一句 which contains... 与第三句 that were sent... 的功能差别。
which contains... 是非限制性补充信息，删去后仍指同一个 archive；that were sent... 是限制性信息，用来从所有 letters 中筛出写给 party officials 的那一类。前者补充，后者分类。
高阶语境：学术科研
第二句冒号后的 that 从句为什么不是普通定语从句？
它解释 a simpler possibility 的具体内容，即“这个可能性是什么”。它不是修饰一个名词的属性，而是填充该名词的命题内容，因此是同位内容从句。
高阶语境：真实生活
用 those addressed to... 模仿写一句，比较两类消息。
可以写：The messages sent to colleagues sound formal, whereas those addressed to close friends are brief and playful. those 代替 messages，addressed to close friends 后置修饰。
先确定名词是否已可识别，再问后置信息是否筛选集合。随后检查从句内部是否留下空位：有空位常是关系从句，成分完整则可能是内容从句。