在英语中，定语从句是用来修饰名词或代词的从句，被修饰的名词或代词称为先行词。定语从句的引导词（如who, whom, which, that, as, whose等）在从句中代替先行词并充当一定的成分，这些成分通常包括主语、宾语、表语、定语或状语等。

判断定语从句中先行词所充当的成分，主要看从句中缺少什么成分。例如：

先行词在从句中作主语

The book which I am reading is very interesting.

The person who called you is my friend.

先行词在从句中作宾语

This is the factory which/that we visited last week.

I met the teacher whom you recommended.

先行词在从句中作定语

The girl who has a red dress is my sister.

The house whose windows are open is empty.

先行词在从句中作状语

This is the place where I was born.

She arrived at the station at 3 o'clock, when the train was leaving.

通过分析从句中缺少的成分，我们可以选择合适的关系词来引导定语从句，并确保其在从句中充当正确的成分。这样不仅可以使句子结构清晰，还能准确表达意思。