一、作人稱代詞，可用來代替人、物或事�！猈ho is knockingat the door?—It‘s me誰在敲門？是我 。

　　The ant is not gathering this food for itself alone. Ithas two stomachs in its body.螞蟻不只 是為自己采食。 它的身體里有兩個(gè)胃。

　　When service is interrupted, as it was by last night‘sstorm, John has to inform the pu blic what the company isdoing about the problem.每當(dāng)供電中斷時(shí)，就像昨夜因暴風(fēng)雨而造成的那樣 ，約翰就必須通知大家，公司為解決這個(gè)問題正在做些什么。

　　二、作非人稱代詞，表示時(shí)間、天氣、季節(jié)、距離、環(huán)境等。

　　It is tea time. Mrs Tuner goes into the kitchen.到了喝茶的時(shí)間了，特納夫人走進(jìn)了廚房。

　　It was winter. it was ten degree below zero.現(xiàn)在是冬天，氣溫是零下10度。

　　It is only half an hour walk to the tall tower. 只有半小時(shí)的路程就可以到達(dá)高塔了。

　　It will be lovely in the park today.今天公園里一定很好玩。

　　三、作先行詞。

　　It is fun for her to study, but it is difficult to studyand work at the same time.對(duì)于她來說學(xué) 習(xí)是有趣的，可是要邊工作邊學(xué)習(xí)對(duì)她來說是困難的。

　　It is necessary for them to get an electrician to do thework for them.對(duì)他們來說請(qǐng)一位電工來為他們做這項(xiàng)工作是必要的。

　　It will be no good learning without practice. 學(xué)習(xí)而不實(shí)踐是不好的。

　　It is no use talking to him about it.和他談這事沒用。

　　It is possible that they‘re finished the bridge.他們完成這座橋是可能的。

　　It is important that we should learn from each other andhelp each other.我們應(yīng)當(dāng)互相學(xué)習(xí)，互相幫助，這是很重要的。

　　2.作形式賓語。這種情況通常只用于能帶以名詞或形容詞作賓語補(bǔ)語的動(dòng)詞后面。

　　In the 1870‘s, when Marx was already in his fifties, hefound it important to study the si tuation in Russia, so hebegan to learn Russian.在19世紀(jì)70年代，馬克思已經(jīng)五十幾歲了，他覺得研 究俄國(guó)的形勢(shì)很重要，便開始學(xué)習(xí)俄語。

　　This has made it necessary for agriculture and industryto develop very quickly.這就使得工 農(nóng)業(yè)必須飛速發(fā)展。

　　四、構(gòu)成強(qiáng)調(diào)句型。為了強(qiáng)調(diào)句子的某一成分，把引導(dǎo)詞“it”用在句首，這種強(qiáng)調(diào)句 的結(jié)構(gòu)是“It is （was ）＋所強(qiáng)調(diào)的成分（主語、賓語、狀語）＋that…”，表達(dá)的意思為“是…， 正是…，就是…”�！癷t ”在這種句型中本身沒什么實(shí)際意義。

　　My temperature! It‘s my back that hurts.我的體溫！是我的背痛。

　　It is not only blind men who make such stupid mistakes. 決不只有盲人才犯這樣愚蠢的錯(cuò)誤。

　　這里需要說明的是：如被強(qiáng)調(diào)的是人，則后面用who或that，如被強(qiáng)調(diào)的是物或其他 情況，則用that。

　　It was yesterday that I met Bob in your room. 我就是在你的房間遇見鮑勃的。

　　It was in your room that I met Bob yesterday. 我就是在你的房間遇見鮑勃的。

　　It was Bob that I met in your room yesterday. 我在你房間遇見的就是鮑勃。