并列連詞

when，while，till，until，since…

When he arrives there he will call you.

他到達那兒以后，他會給你打電話。

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

注意

while所引導的從句的謂語動詞只能是延續(xù)性的，不能用點動詞。（如begin，stop…）

When I arrived there， it was raining.

當我到那兒時，天正在下雨。

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

我進屋時，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.

老師教給我們如何做這工作之后，我們才會開始做。

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)爭結束前，他已經在中國了。

=The war was over before he came to China.

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

戰(zhàn)爭結束后，他來到中國。

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習英語以來，我已經學習了兩千多個英語單詞了。

注意

since引導的是一個過去時的句子，說明自當時以來到現(xiàn)在（自從兩年前以來），主句一般要用現(xiàn)在完成時。

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

我一得到足夠的錢，就買它。