A relative clause is a dependent clause that modifies a noun or pronoun in a sentence. It provides additional information about the noun without creating a separate sentence. For example, in the sentence 'The book that I read was interesting,' we have a main clause 'The book was interesting' and a relative clause 'that I read' which tells us more about which book we're talking about. The relative clause is dependent because it cannot stand alone as a complete sentence.
Relative pronouns are words that connect relative clauses to the main clause. The main relative pronouns are who, whom, whose, which, and that. 'Who' is used for people as subjects, 'whom' for people as objects, and 'whose' shows possession. 'Which' is used for things and animals, while 'that' can be used for both people and things. Each pronoun refers back to a noun called the antecedent, creating a clear connection between the clauses.
There are two types of relative clauses: defining and non-defining. Defining clauses, also called restrictive clauses, are essential to the meaning of the sentence. For example, 'Students who study hard succeed' - the clause identifies which specific students. Non-defining clauses, or non-restrictive clauses, add extra information that could be removed without changing the core meaning. In 'My brother, who lives in London, is a doctor,' the clause gives additional information but isn't essential. Notice the commas around non-defining clauses - they're crucial for correct punctuation.
定语从句是英语语法中的重要组成部分，它是用来修饰名词或代词的从句。定语从句在句子中起形容词的作用，能够为主句中的某个名词或代词提供更多的信息。定语从句通常由关系代词或关系副词来引导。
关系代词是引导定语从句的重要词汇。who用来指人，在从句中作主语。whom也用来指人，但在从句中作宾语。whose表示所有关系，既可以指人也可以指物。which专门用来指物。that既可以指人也可以指物，是使用最广泛的关系代词。
定语从句分为限制性和非限制性两种类型。限制性定语从句不用逗号分隔，它提供的信息对于理解句子意思是必要的。比如'努力学习的学生会通过考试'，这里的定语从句限制了是哪些学生。非限制性定语从句用逗号分隔，它提供的是额外的、补充性的信息。比如'汤姆住在这里，他是我的朋友'，这里的定语从句只是补充说明汤姆是我的朋友。
关系代词在定语从句中可以充当主语或宾语。当关系代词作主语时，比如'给你打电话的那个人'中的who，它是动词called的主语，这种情况下关系代词不能省略。当关系代词作宾语时，比如'我买的那本书'中的that，它是动词bought的宾语，这种情况下关系代词可以省略。所以'我买的书'既可以说成'The book that I bought'，也可以说成'The book I bought'。
定语从句在实际英语中应用非常广泛。通过定语从句，我们可以让句子更加准确和丰富。使用定语从句时，要注意几个要点：首先要确定先行词，也就是被修饰的名词；然后选择合适的关系代词；注意定语从句的语序，通常是关系代词加上一个完整的句子结构；最后要区分限制性和非限制性定语从句的不同用法。掌握这些要点，就能够正确地使用定语从句来表达更复杂的思想。
Let's look at common errors in relative clauses and how to fix them. First, avoid double subjects - don't use both a relative pronoun and another pronoun for the same role. Second, choose the correct relative pronoun - use 'who' for people and 'which' or 'that' for things. Third, remember punctuation rules - use commas around non-defining clauses that add extra information. To avoid these mistakes, always check for double subjects, match your pronoun to its antecedent, and use commas appropriately for additional information.