在句子中，主语是谓语所表示动作或状态的执行者。主语一般位于句子开头，且不能省略。但在某些特定的句式里，如：疑问句、倒装句、祈使句、感叹句等，句子主语也可以位于谓语动词之后或省略。能够充当主语的主要有名词(短语)、代词、数词、动名词(短语)、动词不定式(短语)和从句等。
[例1] (2017·浙江11月高考语法填空)．..and the best part of the process is that there's enough room for improvement, ________ means you'll just keep getting better and better.
[分析] which 空格处所填词引导非限制性定语从句并在从句中充当主语，且指代前面整个主句的内容，故应填关系代词which。
[练1] (2018·浙江6月高考语法填空) Many westerners ________ come to China cook much less than in their own countries once they realize how cheap it can be to eat out.
解析： who/that 分析句子结构可知，空格处引导定语从句，修饰先行词“Many westerners”，且在从句中作主语， 所以应用who或that 引导该定语从句。
