无论在托福阅读文章的过程中，或是在SAT文法做题时，很多同学都会对”in which…, at which…”这类定语从句的分析和理解提出问题。今天我们就通过几个从托福和SAT文法原文中提取的句子，快速帮助同学们熟悉这类定语从句。
1.1 为什么需要了解定语从句(Adjective Clause)
定语从句是学习英语时最重要的语法点之一，定语从句的掌握影响大多数托福长难句的正确分析和理解，影响托福写作中句式的复杂性和多样性，同时也影响后续SAT文法学习中最常考的一类题目——句子结构和标点的正确率。
以SAT文法文章中的句子为例，当表达较为简单的意思时，如一些人倾向于买有机食品，这里我们只需要用一个形容词修饰食品就可以了，句子可以写成：
Some people buy organic food.
但是当我们想表达更复杂的信息时，如另外一些人则倾向于食用传统的由杀虫剂和化肥生产出的食品，显然一个形容词已经不能满足句子的表达需求了，这个时候我们就需要用到定语从句，即用一句话来修饰名词，
Others are prone to conventionally grown food, which are usually produced with pesticides and synthetic fertilizers.
通过例句，我们可以看到，此处的定语从句修饰conventionally grown food, 且修饰的这个名词词组在定语从句中充当主语成分，这使得定语从句形成了一个有着完整主谓结构的句子。
划重点：定语从句的作用是用一个有完整主谓结构的句子修饰名词，以此实现更加复杂的信息表达。
1.2为什么定语从句关系词which前需要加介词(in, on, of, at, from, by, with, for…)
前文说过，定语从句是一个有完整主谓结构的句子，修饰的名词要在定语从句中充当一定成分。而我们最熟悉的定语从句形式，是定语从句修饰的名词，在从句中充当主语。如：
Emerging diseases are also the result of human activity in the modern world, which brings more people into contact with more diseases.
例句中的定语从句修饰human activity, human activity在定语从句中作主语，代入定语从句中，从句可以完整地写为human activity brings more people into contact with more diseases.
但是，我们再来看看介词＋which这类定语从句的结构：
Studies have shown that employees are happier, healthier, and more productive when they work in an environmentin which temperatures are carefully controlled.
例句中，定语从句修饰的是名词environment，但是我们却无法把environment这个名词直接代入从句中，因为即使没有environment，定语从句也已经是一个主谓完整的句子了：temperatures are carefully controlled。
所以我们可以这样理解介词＋which这类定语从句：
当定语从句内部已经是一个主谓宾完整的句子时，定从修饰的名词无法直接代入从句中充当成分，所以通过加入一个介词，使修饰的名词能够以合理正确的语法形式代入定语从句中。
因此，例句中environment前加入in之后，我们把定语从句的语序稍作调整，可以完整的写成temperatures are carefully controlled in the environment.
我们再来练习分析几个托福和SAT中的句子：
1) Each furnished room consists of three walls; the fourth wall is a glass panethrough which museumgoers observe.
- the fourth wall is a glass pane.
museumgoers observe through the glass pane.
2) Besides beginning the age, Superman in many respects defined it, becoming the modelon which many later superheroes were based.
- Superman becomes the model.
many later superheroes were based on the model.
3) Newspaper readers also felt themselves part of the public life about which they were reading.
- Newspaper readers also felt themselves part of the public life.
they were reading aboutthe public life.
