when done……中间省略掉了主语代词it 以及其谓语动词is.这是在从句中很常见的语法现象.规律是:处在从句中的主语和谓语如果是代词的话可以省略掉.例如:The suggestions will not take effect unless treated seriously.unless 后面省略掉了they are.
when 引导时间状语从句,整个从句又是个省略句,没有主语,就看外面从句 that the readings are generally more effective when done after the lecture的主语,是the reading,所以就是说when the reading is done. when done等于when adj. done在这里和形容词作用差不多。