定语从句是英语语法中常用的一种句子结构，用来修饰或限制名词或代词的意义。在写作和口语中，我们常常需要使用多个定语从句来表达更为复杂的意思。而为了避免句子过长和重复，我们可以使用合并句子的方法来简化表达。
合并句子的方法有两种：使用关系代词和省略关系代词。使用关系代词的合并句子，需要将多个定语从句中的关系代词合并成一个，而省略关系代词的合并句子，则是在多个定语从句中省略掉重复的关系代词。
下面是一些例子：
- 使用关系代词的合并句子：
- The book that I borrowed from the library, which was recommended by my teacher, and that I haven't finished reading yet, is due tomorrow.
这个句子中有三个定语从句，使用了三个关系代词that, which, that。我们可以将它们合并成一个：
- The book that I borrowed from the library, which was recommended by my teacher, and haven't finished reading yet, is due tomorrow.
- 省略关系代词的合并句子：
- The girl who lives next door, whose parents are both doctors, and who is very good at playing the piano, is going to study abroad.
这个句子中也有三个定语从句，但是都使用了关系代词who。我们可以将其中两个省略掉：
- The girl who lives next door, whose parents are both doctors, and is very good at playing the piano, is going to study abroad.
通过合并句子，我们可以使句子更加简洁、流畅，避免重复和冗长，提高文章的可读性和口语表达的流利程度。
