(1)不會誤解,能夠清楚知道作者描述的是哪一個東西 EX. Tommy brought his dog, which is a golden retriever, to the park. 湯米帶著他的狗,是一隻黃金獵犬,來到公園。 =從前一句可以了解整個段落在說明湯米有養一隻狗,並用補充說明(加上逗號)的方式描述狗的品種。
(2)要說明的東西僅有一個,不會誤解,例如:月亮、太陽等。 EX. My boyfriend lives in Taichung, which is located in the middle of Taiwan. 我男友住在台中,位於臺灣中部。 =臺中在臺灣僅有一個,因此用補述用法敘述。
(3)描述前面主要句子所發生的事情。 EX. Lisa lost her engagement ring, which made her boyfriend angry. 麗莎丟了訂婚戒指,讓男友很生氣。 =which 補充說明前面主要子句的描述,因為把戒指弄丟所以讓男友生氣。
EX. The house which is located at the end of the street has a beautiful garden. 位於街道盡頭的房子有一個美麗的花園。
★以上這句話中,雖然 「which is located at the end of the street」可以被視為補充資訊,但作者選擇不使用逗號來表達這個定語從句。