The boy burst into tears immediately he saw his mother.

男孩一看到他的母亲就哭了。

3. till，until和not...until的用法

①在肯定句中，主句谓语动词必须是延续性动词，意为"某动作一直延续到某点才停止"。

You may stay here till/until the rain stops.

你得在这里呆到雨停。

②在否定句中，主句谓语动词必须是非延续性动词，从句为肯定式，意为"某动作直到某时间才开始"。

He won't go to bed till(until) she returns.

直到她回来他才睡。

③not...until句型中的强调和倒装用法。

It was not until you told me that I had any idea of it.(强调句型)

Not until you told me did I have any idea of it.(not until置于句首，主句要倒装)

4. before和since的用法

①连词before表示"还未......就......"，"不到......就......"，"......才......"，"趁......还没来得及时"。

Before they reached the station, the train had gone.

他们(还没)到火车站前，火车已经开走了。

We hadn't run a mile before he felt tired.

我们跑了还不到一英里他就累了。

Please write it down before you forget it.

趁你现在没忘把它记下来。

②before的句型：It will be＋时间段＋before从句，表示"多久之后才......"。

It will be half a year before I come back.半年后我才回来。

It won't be long before we meet again.

不久后我们将再见面。

③since的句型：it is/has been＋时间段＋since从句。若从句谓语为非延续性动词，表示"自从......有多久"；若从句的谓语为延续性动词，表示"自从结束/完成......起有多久"。

It is three years since the war broke out.

战争爆发以来，有三年了。