please, who would teach me?

你的句子中有個defining relative clause，所以句子內有兩個動詞。

The boy who is riding a bicycle is my brother.

Defining relative clause是用來指定你說的是哪個男孩，不是在打球的一個，也不是在溫習的一個，而是在踏腳踏車的一個。

上面一位回答寫The girl who is beautiful is my sister的問法沒錯，但因為relative clause的句型是to be + adjective，句中的動詞是to be，所以通常的寫法都是The beautiful girl is my sister，不會用relative clause的。

其實佢e句加左個who.係用黎補充解釋番the boy係指緊邊個
who is riding a bicycle <--你可以睇e段為一句
如果你將佢掩住睇都得..者係咁The boy is my brother
所以你可以當你個句當中係有2個sentense係度..
因而會有2個verb...

講埋你知..如果你想好似佢咁第時用黎補充你講緊個個人
比如你想話個個好靚既女仔係度溫習緊..你可以話
The girl who is beautiful is studying

2006-12-09 00:21:03 補充：
唔記得話你聽成句既意思係:The boy who is riding a bicycle is my brother.個個踩緊單車既男仔就係我細佬

The boy who is riding a bicycle = He


The boy who is riding a bicycle is my brother = He is my brother.

The boy( who is riding a bicycle )is my brother , the 2nd "is" means 係