23.He failed again just　　he didn't work hard at his lessons.

解析：此为because引导的原因状语从句。句意：他再次失败了，因为他没有努力学习。

答案：because

22.I will take you shopping　　it doesn't rain this afternoon.

解析：句意：如果今天下午不下雨，我就带你去逛商店。if引导条件状语从句。

答案：if

21.　　you understand the rule，you will find it very easy to obey.

解析：once此处引导条件状语从句，意为“一旦”；从句意为一旦你理解了规则……。

答案：Once

19.It was not yet 7 o'clock　　he came back.

解析：此为when引导的时间状语从句。when当……的时候。句意：他回来时，还未到七点。

答案：when

you imagine how long it will be　　a new drug is invented for AIDS？

解析：此为before引导的时间状语从句。此为结构It will be+时间+before...；要过多长时间才……

答案：before

18.Please keep reading the poem　　you have remembered it.

解析：此为until引导的时间状语从句，意为“直到”。句意：请不断读这首诗，直到你记住它。

答案：until

17.I didn't know what happened.He had rushed out of the room　　I could ask him.

解析：此为before引导的时间状语从句。before在……之前；还没来得及。句意：我不知道发生了什么事情。我还没来得及问，他就已经冲出了房间。

答案：before

16.　　he is a newcomer，he has few friends around here.

解析：此为because引导的原因状语从句，意为“因为”。句意：因为他是新来的，他在这有很少的朋友。

答案：Because

15.I found all the doors open and everything in disorder　　I arrived home.

解析：考查时间状语从句。as soon as/when/the moment常译为“一……就……”。句意：我一回到家就发现所有的门都开着，一切东西都乱七八糟的。

答案：when/as soon as/the moment

14.The baby may be saved　　he is sent to hospital in no time.

解析：此为if/so long as引导的条件状语从句。if如果；so long as只要。句意：那个婴儿可能会被救活，如果/只要他及时被送到医院。

答案：if/so long as

13.Tom didn't go to bed　　his mother came back from work.

解析：此为until引导的时间状语从句。not...until直到……才。句意：直到他妈妈工作回来，Tom才上床睡觉。

答案：until