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介詞＋which＝when/where�
A.
先行詞是表示時間的名詞（如 time, day, year, century 等）時，介詞必須放在做其受詞的關係代名詞 which 之前，不可放在子句之末。而此一 "介詞＋which" 可以用關係副詞 when代替，也可以用that充當關係副詞代替when。而且，此 when 或 that均可省略。如：
(1)
a. I'll never forget the day which I first met you on. (×)
b. I'll never forget the day on which I first met you. (O)�
c. I'll never forget the day when I first met you. (O)
d. I'll never forget the day that I first met you.
e. I'll never forget the day I first met you.
（我永遠不會忘記我初次和你見面的那一天。）
或問：可否將句中的that視為關係代名詞而在that之前加上介詞on?不行，因為關係代名詞that做介詞的受詞時，此一介詞必須放在子句之末。既然如此，就放在子句的末尾好了，也不行，請再看看 "(1)" 例上面的說明。
(2)
a. July is the month in which the weather is usually the hottest.
b. July is the month when the weather is usually the hottest.�
c. July is the month that the weather is usually the hottest.
d. July is the month the weather is usually the hottest.�
（七月通常是天氣最熱的月份。）

B.
先行詞是表示 "地方" 的名詞（如 room, house, city, country）時和上述 "A" 有兩點不同：其一是介詞也可以放在子句的末尾，其二是that只可視為關係代名詞，不可充當關係副詞，因此以下各例中 that子句裏的介詞都不可遺漏。如：
(1)
a. The city in which we spent our vacation was very beautiful.�
b. The city which we spent our vacation in was very beautiful.
c. The city that we spent our vacation in was very beautiful.�
d. The city we spent our vacation in was very beautiful.
e. The city where we spent our vacation was very beautiful.�
（我們度假的那個城市美不勝收。）
注意：用了where，子句裏不可有介詞 in，下例的 at 也如此。
(2)
a. That is the restaurant at which I will meet him.�
b. That is the restaurant which I will meet him at.�
c. That is the restaurant that I will meet him at.
d. That is the restaurant I will meet him at.
e. That is the restaurant where I will meet him.
（那就是我要與他會面的那家餐館。）�
或問：為什麼時間名詞之後的that只可充當關係副詞而地方名詞之後的that必須視為關係代名詞呢?是根據什麼訂出這雙重標準呢?到目前為止，我還沒有弄清楚! 問了許多洋人，若非回答 "I don't know why. I just know how."， 即是回答 "Usage is everything."。