定语从句的引导词主要分为两类： 关系代词和 关系副词，其选择依据和功能有所不同。以下是具体分析：
一、关系代词（Relative Pronouns）
关系代词用于替代先行词（被修饰的名词或代词）在定语从句中充当成分，主要分为以下几类：
that - 适用范围：人或物
- 功能：可作主语、宾语或表语
- 例：
- The book that
I borrowed is fascinating. [先行词：book，作宾语]
- The man thathelped me is my friend. [先行词：man，作主语]
which
- 适用范围：物
- 功能：作主语或宾语（作主语不可省略）
- 例：
- The car whichbroke down was parked outside. [先行词：car，作主语]
- The letter whichI received was very important. [先行词：letter，作宾语]
who
- 适用范围：人
- 功能：作主语或表语
- 例：
- The teacher whoteaches English is very patient. [先行词：teacher，作主语]
- The person whocalled you last night is my neighbor. [先行词：person，作主语]
whom
- 适用范围：人
- 功能：作宾语（可省略）
- 例：
- The friend whomI met last week is a doctor. [先行词：friend，作宾语]
- The letter whomyou sent arrived yesterday. [先行词：letter，作宾语]
whose
- 适用范围：人或物
- 功能：作定语（表示所属关系）
- 例：
- The book whosecover is blue is mine. [先行词：book，作定语]
- The company whoselogo you saw is global. [先行词：company，作定语]
二、关系副词（Relative Adverbs）
关系副词用于修饰整个定语从句，表示时间、地点或原因，主要分为以下几类：
when
- 适用范围：时间
- 功能：作时间状语
- 例：
- The meeting whenwe discussed the project started at 3 PM. [先行词：the meeting，作时间状语]
- The song whenit started made everyone dance. [先行词：the song，作时间状语]
where
- 适用范围：地点
- 功能：作地点状语
- 例：
- The house wherewe grew up is now a museum. [先行词：the house，作地点状语]
- The event wherethe award was given was very exciting. [先行词：the event，作地点状语]
why
- 适用范围：原因
- 功能：作原因状语
- 例：
- The project whyit was delayed was due to the weather. [先行词：the project，作原因状语]
- The reason whyhe left was unexpected. [先行词：the reason，作原因状语]
三、选择原则
先行词是物： 优先考虑 that
先行词是人：优先考虑 who或 that（作主语/表语）
从句缺成分：
时间/地点/原因：用 when/where/why
宾语：关系代词可省略（如 that which）
特殊结构：
不定代词（如 anything）或被 first/last等修饰时，只能用 that引导
四、注意事项
非限制性定语从句：用 as引导（如 the book as I know），与主句之间用逗号隔开
省略规则：关系代词作宾语时可省略，但需通过上下文明确指
