中考英語知識點(diǎn)：when, while和as的區(qū)別

　　when引導(dǎo)的從句的謂語動詞可以是延續(xù)性的動詞，又可以是瞬時動詞。并且when有時表示"就在那時"。

　　When she came in, I was eating.她進(jìn)來時，我在吃飯。(瞬時動詞)

　　When I lived in the countryside, I used to carry some water for him.當(dāng)?shù)淖≡谵r(nóng)村時，我常常為他擔(dān)水。(延續(xù)性的動詞)

　　We were about to leave when he came in.我們就要離開，就在那時他進(jìn)來了。

　　While引導(dǎo)的從句的謂語動作必須是延續(xù)性的，并強(qiáng)調(diào)主句和從句的動作同時發(fā)生(或者相對應(yīng))。并且while有時還可以表示對比。例如:

　　While my wife was reading the newspaper, I was watching TV. (was reading是延續(xù)性的動詞，was reading和was watching同時發(fā)生)

　　I like playing football while you like playing basketball.我喜歡踢足球，而你喜歡打籃球。(對比)

　　As表示"一邊…一邊"，as引導(dǎo)的動作是延續(xù)性的動作，一般用于主句和從句動作同時發(fā)生;as也可以強(qiáng)調(diào)"一先一后。

　　We always sing as we walk.我們總是邊走邊唱。(as表示"一邊…一邊")

　　As we was going out, it began to snow.當(dāng)我們出門時，開始下雪了。(as強(qiáng)調(diào)句中兩個動作緊接著先后發(fā)生，而不強(qiáng)調(diào)開始下雪的特定時間)

　　有時這三個連詞可以互換，有時不可以。

　　lt was raining hard when (as) I got there.

　　我到那里時，正在下大雨。 ( 動作同時發(fā)生，when可換為as, 但不能換為while，因?yàn)間et是點(diǎn)動詞.)

　　When I had read the article, he called me.

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