when，while，till，until，since…

When he arrives there he will call you.

他到达那儿以后，他会给你打电话。

when， while， till， until， since， after， before， as soon as等是连接时间状语从句的连词。在时间状语从句中要特别注意时态的搭配：当主句是将来时，从句要用一般现在时。

注意

while所引导的从句的谓语动词只能是延续性的，不能用点动词。（如begin，stop…）

When I arrived there， it was raining.

当我到那儿时，天正在下雨。

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

我进屋时，李明正在和她谈话。

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

直到我做完作业，我才上床睡觉。

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

老师教给我们如何做这工作之后，我们才会开始做。

He came to China after the war was over.

比较

连词连接从句不同，意思不同。

He had been in China before the war was over.

战争结束前，他已经在中国了。

=The war was over before he came to China.

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

战争结束后，他来到中国。

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

自从两年前我开始学习英语以来，我已经学习了两千多个英语单词了。

注意

since引导的是一个过去时的句子，说明自当时以来到现在，主句一般要用现在完成时。

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

我一得到足够的钱，就买它。