Luckily, we’d brought a road map without which we would have lost our way.

The book was written in 1946, since when the education system has witnessed great changes.（那本书写于1946年，从那以来教育体制发生了很大变化。）

  本句中when指代前面的时间1946，我们也可以用which time来指代，因此since when=since which time

Since when has the country been open to international trade? （该国是从何时开始一直实行国际贸易开放政策的？）

Since when have you ever been concerned about my feelings?

Since when do you have the right to tell me what to do?（你从什么时候开始有权利这么支使我了？）

Some people are sitting under the big tree, from where they can enjoy the whole view.（有人正坐在那棵大树底下，从那里他们能够欣赏到全景。）

一方面，这里的where指代介词短语under the big tree，因而，from where相当于说from under the big tree。另一方面，整个from where短语在定语从句中充当地点状语，修饰动词enjoy，表明enjoy的地点。因此，定语从句可以改写成这样一个简单句：

  b.They can enjoy the whole view from under the big tree.

  或者，from where相当于笼统的from there，因此把定语从句改成相应的简单句是：

  c.They can enjoy the whole view from there.

  为什么会出现from where这种结构的定语从句呢？我思考后，认为主要是下面两方面原因：

  a.原因分析一：结构上的原因——from与介词短语搭配

  根本原因之一是介词from的用法比较特殊，其后可以再接一个介词短语，即英文中可以说“from+介词短语”。比如本节开头的那句话：

  Human happiness and human satisfaction must ultimately come from within oneself.

  我们看到，from within oneself是由from与介词短语within oneself构成的。

Natural gas is gas used for cooking and heating, taken from under the earth or from under the sea.

I climbed to the top of the tower, from where I could see the whole city.

Natural gas is gas used for cooking and heating, taken from under the earth or from under the sea.

Soon after getting off his horse, the captain appeared at the second storey window, from where he could see nothing but trees.

The captain appeared at the second storey window, where he ran into his wife.（上尉出现在二楼的窗口旁，碰巧在那里撞见了他妻子。）

At the age of 22, John went to a business college in London from where he returned with a Business Diploma certificate.（约翰22岁那年去了伦敦一所商学院求学，后来从该商学院拿到商学文凭证书毕业归来。）定语从句可以改写成这样一个简单句：

  b.He returned with a Business Diploma certificate from a business college in London.