指代性it一般是指人稱代詞it， it作人稱代詞時(shí)可指代事物、動(dòng)物、嬰兒、未知的人或一件事。

　　例句1：It’s a theory to which many economists subscribe, but in practice it often leaves railroads in the position of determining which companies will flourish and which will fail. 這句話中第二個(gè)it就是指代前面的theory。這是一個(gè)許多經(jīng)濟(jì)學(xué)家贊同的理論，但是在實(shí)際中，它往往會(huì)使得鐵路處于一個(gè)決定哪些公司會(huì)興旺，哪些公司會(huì)衰敗的地位中。

　　例句2： Until these issues are resolved, a technology of behavior will continue to be rejected, and with it possibly the only way to solve our problems. 在這句話中，it指代a technology of behavior will continue to be rejected 這一情況。除非這些問(wèn)題得到解決，研究行為的技術(shù)手段就會(huì)繼續(xù)受到排斥，解決問(wèn)題的唯一方法也可能隨之受到排斥。

　　第二種情況是非指代性it。非指代性it是相對(duì)于作人稱代詞的it而言的。他們往往沒有具體意義上的主語(yǔ)，它用于表示氣候、溫度、時(shí)間、地點(diǎn)、距離等。非指代性it常作seem, appear, look, happen, occur, follow等動(dòng)詞的主語(yǔ)，構(gòu)成特定的句型。

　　例句：From an analysis of the hot-spot population it appears that the African plate is stationary and that it has not moved during the past 30 million years.這句中it沒有實(shí)際意義，為非指代性用法，that引導(dǎo)表語(yǔ)從句。從熱點(diǎn)人口分析中可以看出，非洲板塊是靜止不動(dòng)的，而且在過(guò)去的三百萬(wàn)年中也從未移動(dòng)過(guò)。

　　當(dāng)然，在考研考試中，it的重難點(diǎn)更加具體和明確。

　　it 做先行詞。it做形式主語(yǔ)或者形式賓語(yǔ)。It作形式主語(yǔ)時(shí)，往往放在句首，將真正的主語(yǔ)(動(dòng)詞不定式、動(dòng)名詞、從句)放在之后。其中it可作形式主語(yǔ)指代不定式，這時(shí)常見的謂語(yǔ)有：take, pay, make, cost, require, feel, annoy, need等。另外，it形式主語(yǔ)指代It形式主語(yǔ)指代從句，而且主要形式為It is+a.+than; It is+n.+that; It is+done+that; It+vi+that;

　　例如：It is difficult to the point of impossibility for the average reader under the age of forty to imagine a time when high-quality arts criticism could be found in most big-city newspaper.

　　It做形式賓語(yǔ)是放在謂語(yǔ)動(dòng)詞與賓語(yǔ)補(bǔ)足語(yǔ)之間，指代不定式、動(dòng)名詞、從句等。后接it作形式賓語(yǔ)的謂語(yǔ)動(dòng)詞有：feel, consider, find, believe, make, take, imagine, suppose, think, deem, regard, count等。

　　引導(dǎo)強(qiáng)調(diào)句的it。同學(xué)們所熟悉的強(qiáng)調(diào)句型中，它的基本形式就是“it+be的某種形式+that/who從句”。當(dāng)被強(qiáng)調(diào)部分是人時(shí)須用who, 其他情況用that。例句：Curbs on business-method claims would be a dramatic about-face, because it was the Federal Circuit itself that introduced such patents with its 1998 decision in the so-called State Street Bank case, approving a patent on a way of pooling mutual-fund assets. 在because從句中就運(yùn)用了it was ….that的強(qiáng)調(diào)句型，強(qiáng)調(diào)部分為the Federal Circuit itself. 對(duì)商業(yè)方法專利申請(qǐng)的限制將會(huì)是一個(gè)巨大的改變，因?yàn)檫@些專利正是聯(lián)邦巡回法院自己在1998年審理一個(gè)被稱作“美國(guó)道富銀行”的案件時(shí)提出的，判決中巡回法院批準(zhǔn)了一項(xiàng)共同基金資產(chǎn)集資方法的專利。