首先咱们还是得把“定语从句”这个概念分成两部分来理解，分别是“定语（modifier）”和“从句（dependent clause/subordinate clause）”。进一步讲，定语是“功能概念”，而从句是“表现形式”。
大家或许还记得本系列的第一期我讲的是doing作修饰（记不得的回去看看好不？），当时我就说doing可以作修饰，只是大部分学生都不太适应而已。
换句话说，英语中能有修饰功能（也就是定语）的形式不止一种，小到形容词，大到介词短语和非谓语，亦或是今天要说的定从，都可以在一句话内起到修饰和限定的作用。
再说说从句，这个“从”代表着“从属”，具体来说，一套从句结构可以看作是“小句子从属于大句子”，或者，“小句子是大句子的一部分”。其英文概念中的dependent或subordinate两个单词的含义也说明了类似的关系。
所以，定从就是“定语变成了小句子”。
以上我只是把定从的基本概念解释清楚了，但是围绕着它还有数不清的谜团和令人抓狂的语法规则，我一点不是夸张！
★ 随便举几个例子：
限定性定从（restrictive attributive clause）和非限定性定从（non-restrictive attributive clause）你分得清嘛？
定从里面这个that或which什么时候能被省略？又什么时候绝对不能省？它们到底又在指代谁？
除了that或which这两个最常见的，其他的连接词（introductory word）又该如何选择，比如whom，when，where，whose，in which，甚至是against which！？
定从和“同位语从句（appositive clause）”在外形上如此之像，它们到底啥区别啊？
定从里面再给套个插入语（parenthesis），那将会是怎样一副画面啊。。。？
从本期开始，本系列将进入定从的全面解析，力求在经典语法规则上助力各位考生有所突破。这次则先从第一个问题开始，即：怎么区分限定性定从和非限定性定从。
★大伙儿先做一道连线题：
There are two apples which are red on the table.
There are two apples, which are red, on the table.
◾ 正确答案就是：第一个句子对应第一张图，第二个句子对应第二张。
具体来说：
第一张图明明有很多苹果，但是我却只描述成了“two apples”，原因是因为我有个条件：“which are red”，言外之意，第一个句子里，从句的描述真正对于苹果数量有了限制（很多个被限定成两个），具有一种“排他性”。
第二张图则简单许多，就俩苹果，只不过恰巧它俩都是红色的而已，所以which are red没有限制苹果的数量，只是补充说明，没有“排他性”。
总结起来，第二句话里的从句部分对于整体句意没太大影响，而只是让其更丰富一些，去掉也没太大关系。而第一句话里面的从句部分则严重影响着整个句意，因为如果你把它去掉，整个第一句就变了意思，就对不上第一张图了。
说到这里，小部分长得帅的同学可能已经发现了，限定和非限定其实外在上就只是有没有逗号的差别啊！Then you got it！
这个则可以对接到考点：逗号的有无（necessary or unnecessary comma）。
★直接上题了嗷！
1. The Great Boston Molasses Flood of _______ killed 21 people and injured 150.
A) 1919, which moved faster than one would expect,
B) 1919, which moved faster than one would expect--
C) 1919 which moved faster than one would expect
D) 1919 which moved faster than one would expect
2. Artists such as these Brooklyn performers find that crowdfunding exacerbates problems that already exist. _______ while more complex work goes unnoticed.
A) Work, that is easily understood and appreciated is supported,
B) Work that is easily understood and appreciated is supported,
C) Work that is easily understood, and appreciated is supported
D) Work, that is easily understood and appreciated, is supported,
3. The advantages of growing up in a bilingual home can start as early as six months of age. A study conducted by York University’s Institute of Health found that _______ show better attentional control than infants being raised in monolingual environments. The findings suggest that early exposure to multiple languages could set the stage for lifelong cognitive benefits.
A) infants, who are regularly exposed to more than one language,
B) infants who are regularly exposed to more than one language,
C) infants, who are regularly exposed to more than one language
D) infants who are regularly exposed to more than one language
正确答案：ABD。依次分别是：非限定性，限定性，限定性。不明白这个结论的可以再次尝试把定从部分去掉，你会发现只有第一题原句句意不受严重影响！
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