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He was doing his homework when I came in.（當(dāng)）我進(jìn)來(lái)時(shí)，他正在做家庭作業(yè)
I came in when/while he was doing his homework.當(dāng)他在做家庭作業(yè)時(shí)，我走了進(jìn)來(lái)
While/When I was in the town，I saw him twice.（當(dāng)）我在城里時(shí)，見(jiàn)過(guò)他兩次
Strike while the iron is hot．趁熱打鐵。．
when還可譯作這時(shí)while還可譯作“而”
He was playing football outside when it began to rain.
他在外面踢足球，這時(shí)開(kāi)始下起雨來(lái)。
He is tall while his elder brother is short.
他個(gè)子高，而他哥哥個(gè)子矮
2. because，since.和as
because引導(dǎo)的從句通常表示說(shuō)話人認(rèn)為聽(tīng)話人不知道的原因或理由，從句往往是整個(gè)主從復(fù)合句的重心。
He was annoyed because he missed the bus. 他感到為難是因?yàn)樗麤](méi)有趕上那班車
since和as表示原因時(shí)，往往表示人們顯然已知的理由或原因；因此，側(cè)重點(diǎn)是主句的內(nèi)容，原因只作附帶說(shuō)明。在語(yǔ)氣上，becausc最強(qiáng)，since次之（在意義上相當(dāng)于漢語(yǔ)中的“既然”),as最弱
Since his method doesn‘t work，let‘s try yours. 既然他的方法不起作用，我們就試一試你的。
As she was in a bad mood，we left her alone. 由于她心情不好，我們不去打擾她了。
He is absent frorn school because he is ill. 因?yàn)樗�，所以他沒(méi)來(lái)上課
Because/As it was wet，we went there by bus. 因?yàn)橄掠辏晕覀兂塑嚾ツ莾骸?
Since everybody is here，let‘s begin. 既然大家都到了，我們就開(kāi)始吧。
3.if和unless
引導(dǎo)條件狀語(yǔ)從句的連詞，主要是if和unless,由if條件狀語(yǔ)從句可以是真實(shí)的條件句，也可以非真實(shí)條件句．真實(shí)條件句所表示的條件指的是事實(shí)，即可以是實(shí)現(xiàn)的事，非真實(shí)條件句指的是與事實(shí)相反或不大可能實(shí)現(xiàn)的事．
If it rains, the match will be cancelled（取消）． 如果天下雨，比賽將被取消
If I were you, l should consult（請(qǐng)……看�。゛ doctor. 我要是你，我就去看醫(yī)生
If I had tried hard last term，I would have succeeded. 如果上學(xué)期我努力的話，我現(xiàn)在就已經(jīng)成功了
unless在意義上相當(dāng)于if... not
You will fail unless you work harder. 除非你努力，否則你不會(huì)成功。
