首先看两个句子：
She is my girlfriend who works in the hospital.
翻译：这就是我那个在医院工作的女朋友。
暗含：我不只有一个女朋友！这只是其中的一个！
She is my girlfriend, who works in the hospital.
翻译：她是我的女朋友，她在医院工作。
暗含：我只有一个女朋友！后面的定语从句起补充说明的作用！
区别：加逗号定语从句 vs 不加逗号定语从句
不加逗号定语从句（限制性定语从句）：前面的n.是一个“泛指”的笼统的概念！ 多选一！
加逗号的定语从句（非限制定语从句）：前面的n.是一个“具体”的明确概念！从句只是补充说明！
再各举两个例子感受一下什么n.是笼统，什么是具体：
笼统名词：
There are many computer games which contain lots of violent scene that is harmful to children.
We have several methods, such as Television and Internet, to get information. These are resources which can be easily had access to.
具体名词：
Students can develop a sense of belonging and experience team spirit, both of which may prove very important in building a career.
Harry Potter I, which is written by J.K.Rowling, is popular.
补充：“加逗号”的定语从句位于句中 vs 位于句尾
位于居中举例：
原句：He moved to our city two years ago. He bought a new car last week.
修改：Tom, who moved to our city two years ago, bought a new car last week.
说明：定语从句位于居中，可以连接一个人的两个动作；其中“不太重要”的动作成为中间的定从！
位于句尾举例：
More and more people come to the ancient village during the holidays, which boosts the local economy.
说明：定语从句位于句尾，可以对前文进行“小结”，说明带来的情况！
练习一个翻译：汉朝是中国历史上最重要的朝代之一，汉朝统治期间有很多显著的成就。
翻译：
The Han Dynasty, which is one of the most significant dynasties in the history of China, attained lots of remarkable achievements during the reign.