【问】 The function of Louis Sullivan's architecture was
 large uninterruptedand to allow(TOEFL，1980.5)
 试题要求将动名词providing改为动词不定式to provide。但是我不理解为什么要作这种更改。难道动名词不可以充当表语(主语补语)吗?请看下面的例句：
 1. My hobby is swimming.
 我的爱好是游泳。
 2. Her first job had been selling computers.
 她的第一个工作曾是销售计算机。
 3. Seeing is believing.
 百闻不如一见。
 4. The only thing that interests him is dancing.
 他唯一感兴趣的事是跳舞。
 【答】 知道动词不定式和动名词都可以充当表语(主语补语)，这只是个一般的知识。在这个基础上还需要一些更具体的知识。比如，在the fuction充当主语时，其表语(主语补语)只能由动词不定式来充当，而不能由动名词来充当。请看下面的实例：
 1. “Did the audience participate in the play?”
 “Yes，those actors________to involve the audience. ”
 A. whom it was the function B. of whom the function was
 C. whose function it was D. whose were the function(TOEFL，73. 6)
 2. The main function of the kidneys is to purify the blood. (FWF)
 肾的主要功能是净化血液。
 3. The function of an adjective is to describe or add to the meaning of a noun. (LDCE)
 形容词的作用是描述或增加名词的意义。
 4. The function they perform is to maintain the established order of society. (London)
 他们的职务是维护建立起来的社会秩序。