當心定語從句惹禍

請看下面一道題：The letter they were looking forward to _________ at last.
A. arrived                 B. arriving                C. had arrived           D. arrive
對於此題，基礎稍差的同學可能選D，認爲空格前的 to 是不定式符號，所以後接動詞原形；基礎稍好的同學可能會選B，知道 look forward to 是短語，意爲 look forward to “盼望”，其中的 to 是介詞，而不是不定式符號，所以後接動名詞；只有基礎紮實、語感好、而且細心的同學纔會選A，誠如上面所說，look forward to 中的 to 是介詞，而不是不定式符號，所以後接動詞作賓語時，要用動名詞，而不用動詞原形。但問題是此句中的動詞 arrive 不是介詞 to 的賓語，而是句子的謂語。句子的正解分析是：the letter 爲句子主語，they were looking forward to 是修飾主語的定語從句(介詞 to 的賓語是引導該定語從句的關係代詞 that，被省略)，arrived 是句子謂語。請看以下試題：

(1) The professor you referred to _________ just now.
A. comes                    B. come                   C. coming                    D. came                   
答案選D，句子主語爲 the professor，you referred to 爲修飾 the professor 的定語從句，空白處填 came，爲句子謂語。

(2) The theory he sticks to _________ to be of no use in our studies.
A. proves                   B. prove                    C. proving                   D. be proved 答案選A，he sticks to 是修飾主語 the theory 的定語從句，句子的基本結構爲 The theory proves to be of no use in our studies.

(3) The work he devoted his time to _________ worth praising.
A. was                      B. be                    C. being                    D. been
答案選A，he devoted his time to 是修飾主語 the work 的定語從句。句子的基本結構爲 The work was worth praising.