it做形式主语或形式宾语
为了使句子结构更加清晰对称，经常会用it代替不定式、动名词短语或从句在句中充当形式主语，而将真正的主语置于句子后面。当动名词或不定式充当动词宾语时，后面又跟有宾语补足语时，须用it充当形式宾语，而将真正的宾语（动名词或不定式）置于宾补之后。
关于it做形式主语的用法，有很多常见的句式：
1.It is done that…
It is believed that…据认为……，人们相信……
It is considered that…有人认为……
It is reported that…据报道……
It is said that…据说……
It is supposed that…据推测……
It is told that…据传……
It is well known that…众所周知……
It is estimated that…据估计……
It was pointed out that…有人指出……
It is generally accepted that…人们通常认为……，一般认为……，大家公认……
It is alleged that…人们断言……
It is claimed that…有人主张……，人们要求……
It cannot be denied that…不可否认
It has been found that…实践证明
It is predicted that…据估计……，据预测……
It is suggested that…有人建议……(https://www.daowen.com)
2.It is+adj.+（of/for sb.）to do sth./that…
该句型中，it为形式主语，真正的主语为后面的不定式或that从句。如：It is very impolite to speak very loudly in public.在公共场合大声说话是很不礼貌的。
当形容词为sb.本身具有的性质、特点时，用of，像kind，nice，clever，foolish等；当形容词是表示事物的性质时，则用for，像difficult，impossible，interesting等。
如：It’s silly of you to ask such questions.你提出这样的问题，实在是太傻了。（表示“你真傻”）
It is necessary for us to sleep well.睡眠好对我们来说是必需的。（表示“睡眠是必需的”）
3.It is necessary/important/advisable/…that…
与表示重要性、必要性、公平性等含义的形容词相连的名词性从句一般要采用虚拟语气，其谓语动词用should do的形式，should常省略。常见的这类词语包括：important，necessary，essential，unfair，advisable，appropriate，desirable，imperative，impossible，incredible，obligatory，proper，vital，urgent等。如：It is imperative that these safety rules be obeyed by everyone.这些安全规则必须被所有人遵守。
4.It turns out to be/that…
It turns out to be/that…表示“结果是……，原来是……”，强调一种出乎意料的结果，后面可以接to be…，也可以接that从句。
如：It turned out to be one of his old friends.原来是他的一个老朋友。
It turned out that his brother paid the debt for him.原来是他弟弟帮他还了债。
5.It goes without saying that…
It goes without saying that…表示“不言而喻，……”如：It goes without saying that in order to speak good English，we must first of all learn good pronunciation.不言而喻，为了讲好英语，首先必须学好发音。
6.It appears/seems that…
It appears/seems that…表示“看来似乎……，好像……”，如：In fact，it seems that nearly every poet or philosopher in history has tried to define love.事实上，似乎历史上几乎每一位诗人或哲学家都曾试图诠释爱。
例1：汉语：如今，人们经常会看到年轻的女性不再遵守女性语言规范。
译文：Nowadays，it is commonly observed that young women are not conforming to the feminine linguistic ideal.
分析：本句按照英文习惯译成了被动语态，使用了形式主语it。
例2：汉语：乔治从机场给他的老板打电话，但接电话的却是他的助手。
译文：George called his boss from the airport but it turned out that his assistant answered the call.
分析：“却是”表明“结果出乎意料”，故可用it turns out that…句式表达。