首先亲要确定的是when不是介词 when疑问副词、从属连词（名词性从句和状语从句中）、关系副词（定语从句中）
当before 和 after 用作介词时,其 后可以跟非谓语动词（动名词doing）
而当这三个词出现楼主所说的省略这一语法现象时,三者都是从属连词,引导一时间状语（不省略则称作时间状语从句）
例如：When water is heated,water can boil./When heated,water can boil.
When he was playing upstairs,a stranger came to visit his home.
When he was playing upstairs,he broke his buttock.
/When he playing upstairs,a stranger came to visit his home.（从句与主句主语不一致时,从句主语不可省）
/When playing upstairs,a stranger came to visit his home.(从句与主句主语一致时,从句主语谓语可省)
此种愈发现象叫做省略,句法作用不变,仍是状语；名称上,这种没有谓语又能单独表达一个分句的意思的结构叫做独立结构或独立主格结构.涵盖范围很广,基本上状语从句都能转化.此处不赘言了.