一、it作形式主语。
英语中不定式、动名词、主语从句在作主语时主语往往显得很长，使整个句子看上去有点头重脚轻，这样就借助“it”来作形式上的主语，而把真正的主语移到句子后边去，使全句结构上前后平衡。 “it”并无实际意义。It之后的谓语部分，可以是be+形容词，be+名词(词组)，或be+过去分词等。
1.不定式做真正主语
例句1：If you are part of the group which you are addressing, you will be in a position to know the experiences and problems which are common to all of you and it’ll be appropriate for you to make a passing remark about the inedible canteen food or the chairman’s notorious bad taste in ties. (2002 Text1)
【结构分析】本句包含由and连接两个并列分句。第一个分句主干为you will be in a position，不定式to know the experiences and problems 做定语，修饰position，其中which引导定语从句，修饰the experiences and problems。第二个分句为it’ll be appropriate for you to …，其中it作形式主语，真正的主语为不定式to make a passing remark about…。
【参考译文】如果你是你谈话听众中的一员，你就能够了解你们所共有的经历和问题，而且你也可对餐厅极难吃的食物或者领导在选择领带方面差劲的品味进行评头论足。
例句2：Just as on smoking，voices now come from many quarters insisting that the science about global warming is incomplete, that it’s OK to keep pouring fumes into the air until we know for sure。(2005 Text2)
【结构分析】本句的主干是voices come from…，现在分词短语insisting…作voice的后置定语，insiste后接两个 that引导的宾语从句，分别为that the science about global warming is incomplete和that it’s OK to keep pouring fumes into the air until we know for sure，第二个that从句中，it是形式主语，真正的主语由不定式短语充当，until we know for sure为时间状语从句。
【参考译文】就像吸烟问题一样，来自不同领域的声音坚持认为有关全球气...