用。

　　5．(2016·天津卷，9改编)We will put off the picnic in the park until next week, __when__ the weather may be better.

　　解析：考查定语从句。句意：我们将把公园的野餐推迟到下个星期，那时天气可能会更好。使用定语从句，先行词是next week，定语从句中不缺少主语、宾语和表语，缺少时间状语。故填关系副词when。

　　6．When she was a small girl, her mother arranged for her to have extra classes on Saturday __where__ piano lessons are taught.

　　解析：考查定语从句。先行词为classes，定语从句piano lessons are taught不缺宾语或主语，所以先行词应该作状语，表地点用where。

　　7．Is this the school __where__ you studied twelve years ago?

　　解析：考查定语从句。句意：这就是你十二年前就读的那所学校吗？本句中先行词为the school，定语从句中缺少地点状语，故用where，相当于in which。

　　8．"We went through a period __when__ communications were very difficult in the rural areas．"said Mo Yan.

　　解析：考查定语从句的关系词。先行词是period，定语从句中不缺少主语或宾语，因此period应该在定语从句中作状语，与介词的搭配是in the period，相当于when。

　　9．Can you think of a situation __where__ this expression can be used?

　　解析：考查关系代词的选用。在这个定语从句中this expression can be used不缺少主语和宾语，所以不可以填which和that或省略，因此断定定语从句中缺少状语，where相当于in a situation。

　　10．We all want to know the reason __why__ you left without a word.

　　解析：考查定语从句。why＝for which，引导reason后的定语从句，在从句中作状语。句意：我们都想知道你不辞而别的原因。