which, where, when, why的用法
首先，我們先介紹一個重要的觀念：介系詞加上名詞或代名詞等於形容詞或副詞。而我們知道，which 是關係代名詞 – 具有連接詞功能的代名詞（想要進一步了解關係代名詞的用法，可以閱讀who, whom, whose的用法分別與that, which, who, whom的用法差異），而where, when 跟why是關係副詞（也就是具有連接詞功能的副詞）。由此可得，介系詞加which可以等於where, when跟why。
利用疑問句了解where, when, why
現在，先來複習一下6Ｗ疑問句。where問動作發生的地點，也就是地方副詞; when問動作發生的時間，也就是時間副詞; 而why問動作的原因。要記得，英文句子結構裡，疑問句的主詞跟動詞要倒裝。
- He lives in New York.
Where does John live?
（where出現，句子的地方副詞就不見） - He visited Japan in 2018.
When did he visit Japan?
（when出現，句子的時間副詞就不見） - He stayed up late for the test.
Why did he stay up late?
（why出現，句子的原因副詞就不見）
where, when跟why當關係副詞
了解where, when跟why代替句子的哪個部分後，我們用他們來造形容詞子句。要注意形容詞子句不是疑問句，主詞動詞不要倒裝。
- He went to Hong Kong. He stayed in Hong Kong for a month.
＝He went to Hong Kong, where he stayed for a month.
（在第二個句子中，in Hong Kong是地方副詞，用where代替，將where移到第二個句子的句首，並且將第二句放到第一句的Hong Kong後面） - She died on July 15th. On the day, all her family except her were out on holiday.
=She died on July 15th, when all her family except her were out on holiday.
（在第二個句子中，on the day是時間副詞，用when代替，因為the day = July 15th，所以將第二個句子放到July 15th後面） - That was the reason. She did not come to your party for the reason.
=That was the reason why she did not come to your party.
（在第二個句子中，for the reason是原因副詞，用why代替，將why移到第二個句子的前面）
複習了以上這些基礎概念後，我們來將which跟where、which跟when、還有which跟why在形容詞子句裡的用法，一一對照說明清楚。
which, in which, where
which是關係代名詞，取代句子中的名詞或代名詞，而變成形容詞子句。
- She put the basket on the table. Her cat lay in the basket.
=She put the basket which her cat lay in on the table.
(which 代替名詞the basket，in可以留在後面）
=She put the basket in which her cat lay on the table.
(in 也可以移到前面）
where則是關係副詞，取代句子中的地方副詞，讓句子變成形容詞子句。
- She put the basket on the table. Her cat lay in the basket.
=She put the basket where her cat lay on the table.
(in the basket是地方副詞，用where代替）
一般來說，只有顯而易見的情況，使用where，（例如in the place, on the chair等），不然，還是用介系詞加上which來表達會比較清楚一點。下面列舉一些不同的介系詞加上which的例子。
- She stood under the tree. They had buried their treasure near the tree.
=She stood under the tree near which they had buried their treasure.
(不用where，因為讀者無法由where這個字，清楚知道位置。） - In the room, there was a window. They put a table at the window.
＝In the room, there was a window at which they put a table.
＝In the room, there was a window where they put a table.
(由前後文可以知道桌子跟窗戶的位置關係，所以可以使用where） - They swam in the river. They put up their tent by the river,
=They swam in the river by which they put up their tent.
（不用where，因為無法清楚表達帳篷跟河的位置關係。） - Her cat lay under the chair. She sat on the chair.
=Her cat lay under the chair on which she sat.
=Her cat lay under the chair where she sat.
(可用where，因為大部分的情況是坐在椅子上.)
which, in which, when
應用同樣的概念，when取代句子中的時間副詞，而which一樣取代句子中的名詞。
- She was born in 1945. World War II came to an end in 1945.
=She was born in 1945, in which World War II came to an end.
=She was born in 1945, when World War II ended. - I like spring. In spring flowers and trees come to life.
=I like spring, in which flowers and trees come to life.
=I like spring when flowers and trees come to life. - I will always remember the day. On the day I lost my daughter.
=I will always remember the day on which I lost my daughter.
=I will always remember the day when I lost my daughter. - I did not know the time. The earthquake hit at the time.
=I did not know the time at which the earthquake hit.
=I did not know the time when the earthquake hit.
which, for which, why
why取代句子中的原因，使用which的話就只有用for which
- That was the reason. She worked so hard for the reason.
=That was the reason for which she worked so hard.
=That was the reason why she worked so hard.
接下來，要請讀者自己多造幾個句子，以便讓自己更熟練這個文法重點了。此外，如果你有其他文法上的疑惑，可以到我們『英文文法』專頁，看看有沒有文章探討相關文法問題。
