并列連詞

when，while，till，until，since…

When he arrives there he will call you.

他到達(dá)那兒以后，他會(huì)給你打電話。

when， while， till， until， since， after， before， as soon as等是連接時(shí)間狀語從句的連詞。在時(shí)間狀語從句中要特別注意時(shí)態(tài)的搭配：當(dāng)主句是將來時(shí)，從句要用一般現(xiàn)在時(shí)。

注意

while所引導(dǎo)的從句的謂語動(dòng)詞只能是延續(xù)性的，不能用點(diǎn)動(dòng)詞。（如begin，stop…）

When I arrived there， it was raining.

當(dāng)我到那兒時(shí)，天正在下雨。

I entered the room while（when） Li Ming was talking with her.

我進(jìn)屋時(shí)，李明正在和她談話。

I didn't go to sleep until（till）I finished my homework.

直到我做完作業(yè)，我才上床睡覺。

We won't work until（till） our teacher teaches us how to do it.

老師教給我們?nèi)绾巫鲞@工作之后，我們才會(huì)開始做。

He came to China after the war was over.

比較

連詞連接從句不同，意思不同。

He had been in China before the war was over.

戰(zhàn)爭(zhēng)結(jié)束前，他已經(jīng)在中國(guó)了。

=The war was over before he came to China.

=The war had been over before he came to China.

戰(zhàn)爭(zhēng)結(jié)束后，他來到中國(guó)。

I have learned more than two thousand English words since I began learning English two years ago.

自從兩年前我開始學(xué)習(xí)英語以來，我已經(jīng)學(xué)習(xí)了兩千多個(gè)英語單詞了。

注意

since引導(dǎo)的是一個(gè)過去時(shí)的句子，說明自當(dāng)時(shí)以來到現(xiàn)在（自從兩年前以來），主句一般要用現(xiàn)在完成時(shí)。

As soon as I get enough money，I'll buy it.

我一得到足夠的錢，就買它。