完整的从句应该是 when he was in prison,当从句主语和主句主语相同的时候,可以省略从句的主语,而将谓语改为分词的形式（举例：After hearing the news,she cried.(未省略的：After she heard the news,she cried.)以及 When taught by Ms Chen,we made good progress.(未省略的：When we were taught by Ms Chen,we made good progress.)）特殊的,如果是主系表结构,可以把主语和系动词都省略.
如果主语不一样,事实上也可以省略,不过这就是一种比较复杂的语言现象,叫做独立主格