有这样一道题，这个 to 的后面该什么呢？

the letter they were looking forward to _________ at last.

a. arrived                        b. arriving                       c. had arrived                    d. arrive

分析：对于此题，基础稍差的同学可能选d，认为空格前的 to 是不定式符号，所以后接动词原形；基础稍好的同学可能会选b，知道 look forward to 是短语，意为“盼望”，其中的 to 是介词，而不是不定式符号，所以后接动名词；只有基础扎实、语感好、而且细心的同学才会选a，诚如上面所说，look forward to 中的 to 是介词，而不是不定式符号，所以后接动词作宾语时，要用动名词，而不用动词原形。但问题是此句中的动词 arrive 不是介词 to 的宾语，而是句子的谓语。句子的正解分析是：the letter 为句子主语，they were looking forward to 是修饰主语的定语从句(介词 to 的宾语是引导该定语从句的关系代词 that，被省略)，arrived 是句子谓语。请看以下试题：

(1) the professor you referred to _________ just now.

a. comes                        b. come                        c. coming                        d. came

答案选d，句子主语为 the professor，you referred to 为修饰 the professor 的定语从句，空白处填 came，为句子谓语。

(2) the theory he sticks to _________ to be of no use in our studies.

a. proves                        b. prove                        c. proving                        d. be proved

答案选a，he sticks to 是修饰主语 the theory 的定语从句，句子的基本结构为 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

答案选a，he devoted his time to 是修饰主语 the work 的定语从句。句子的基本结构为 the work was worth praising.