首先看一个定语从句的例子，请看下面第一句。

　　①He grew up in the house [in where] he still lives with his wife.

　　②He grew up in the house [where] he still lives [in the house] with his wife.

　　③他在这所房子里长大，他和他妻子现在仍然住在这所房子里。

　　我们可以看到，在先行词where的前面，有一个介词in。假如我们不要这个in，那么这句话应当改写成上面第二句，而第二句有一个明显的毛病，那就是 the house出现了两次，一次出现在主句中，一次出现在从句中，而这种同一成分在同一句子中的多次重复，正是英语的大忌，所以我们必须将in转移到 where的前面，从而将第二次出现的the house省略掉，并用where代替这个被省略掉的the house，您明白了吗?下面我再举三个类似的例子，请您自己分析一下，原理都一样，我就不罗嗦了。

　　①This is the book [by which] Newton"s calculus was introduced into China.

　　②This is the book [which] Newton"s calculus was introduced [by the book] into China.

　　③正是通过这本书，使牛顿的微积分介绍到中国来。

　　①Please contact your webmaster [from whom] you can obtain a new password.

　　②Please contact your webmaster [whom] you can obtain a new password [from your webmaster].

　　③请联系您的网络管理员，您可以从他那里获得一个新口令。

　　①This is the ruler [with that] you can draw a straight line.

　　②This is the ruler [that] you can draw a straight line [with the ruler].

　　③这就是那把尺子，您可以用它划一条直线。