我正在看书时，他进来了。

　　The students sang as they walked.

　　学生们边走边唱。

　　As a young man (＝ When he was a young man), he was fond of hunting.

　　他年轻时喜欢打猎。

　　(2)before与since引导的时间状语从句

　　①before与since的常用句式

连词 词义 常用句式 before 在......之前，还未......就......；......才......；趁......，还没来得及...... (1)It will (not) be＋一段时间＋before ..."(没有)过......(时间)才(就)......"

(2) It was not long before ..."不久......就......"

(3) It was＋时间段＋before ..."过了......(时间)才......" since 自从......以来 It is/has been＋一段时间＋since ...(从句用一般过去时) 　　

　　It will be half a year before I come back.

　　我半年之后才回来。

　　It won't be long before we meet again.

　　用不了多久我们就会再见面。

　　It was three days before he came back.

　　三天后他才回来。

　　②since引导的从句如果用非延续性动词，所表示的是自从句谓语动作算起；若用延续性动词或状态动词，所表示的就是自动作或状态的完成或结束时算起。

　　It's two years since we arrived here.

　　我们到这儿两年了。

　　It's three years since we lived here.

　　我们不住在这里有三年了。

　　(3)表示"一......就......"含义的词(短语)引导的时间状语从句

　　①引导时间状语从句，且表示"一......就......"含义的词或短语有as soon as, the moment, the minute, immediately, directly, hardly/scarcely ... when/before, no sooner ... than ...等。

　　Directly you feel any pains, you must go to the doctor.

　　你一感觉疼痛就必须去找医生。

We will leave the minute you are ready.