What is the function of the underlined part?

Tom is a handsome boy. The little boy needs a blue pen. The boy in the classroom needs a pen. The pen bought by her is made in China. The man standing there is my teacher. The boy who is reading needs the pen which you bought yesterday.

单词作定语时通常放在它所修饰的词之前，作前置定语。 短语和从句作定语时则放在所修饰的词之后，作后置定语。

定语从句：一个句子充当定语