一、作人稱代詞，可用來代替人、物或事。
    —Who is knocking at the door? —It's me 誰在敲門?是我。
    The ant is not gathering this food for itself alone. It has two stomachs in its body.
    螞蟻不只 是為自己采食。它的身體里有兩個胃。
    When service is interrupted, as it was by last night's storm, John has to inform the public what the company is doing about the problem.
    每當供電中斷時，就像昨夜因暴風雨而造成的那樣，約翰就必須通知大家，公司為解決這個問題正在做些什么。
    二、作非人稱代詞，表示時間、天氣、季節(jié)、距離、環(huán)境等。
    It is tea time. Mrs. Tuner goes into the kitchen.
    到了喝茶的時間了，特納夫人走進了廚房。
    It was winter. it was ten degree below zero. 現在是冬天，氣溫是零下10度。
    It is only half an hour walk to the tall tower.
    只有半小時的路程就可以到達高塔了。
    It will be lovely in the park today. 今天公園里一定很好玩。
    三、作先行詞。
    1. 作形式主語。英語中不定式、動名詞、主語從句在作主語時主語往往顯得很長，使整個句子看上去有點頭重腳輕，這樣就借助“it”來作形式上的主語，而把真正的主語移到句子后邊去，使全句看上去平穩(wěn)一些?！癷t”沒有實際的意義。
    It is fun for her to study, but it is difficult to study and work at the same time.
    對于她來說學習是有趣的，可是要邊工作邊學習對她來說是困難的。
    It is necessary for them to get an electrician to do the work for them.
    對他們來說請一位電工來為他們做這項工作是必要的。
    It will be no good learning without practice. 學習而不實踐是不好的。
    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 and help each other.
    我們應當互相學習，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語的動詞后面。
    In the 1870's, when Marx was already in his fifties, he found it important to study the situation in Russia, so he began to learn Russian.
    在19世紀70年代，馬克思已經五十幾歲了，他覺得研究俄國的形勢很重要，便開始學習俄語。
    This has made it necessary for agriculture and industry to develop very quickly.
    這就使得工農業(yè)必須飛速發(fā)展。
    四、構成強調句型。
    為了強調句子的某一成分，把引導詞“it”用在句首，這種強調句的結構是“It is (was )+所強調的成分(主語、賓語、狀語)+that…”，表達的意思為“是…， 正是…，就是…”?！癷t ”在這種句型中本身沒什么實際意義。
    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誤。
    這里需要說明的是：如被強調的是人，則后面用who或that，如被強調的是物或其他情況，則用that。
    It was yesterday that I met Bob in your room.
    我就是昨天在你的房間遇見鮑勃的。
    It was in your room that I met Bob yesterday.
    我就是在你的房間昨天遇見鮑勃的。
    It was Bob that I met in your room yesterday.
    昨天我在你房間遇見的就是鮑勃。
    ?
    

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