以定語從句為例來説明這個問題。
The girl who is wearing red coat is my sister.
在這個句子中 主語 the girl 受定語從句 who is wearing red coat 的修飾，謂語是 is，表語是 my sister。the girl 是先行詞，who 就是引導詞，也就是 關係代詞。
再看一個例句:
The man over there who you were just talking with is my brother.
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