1.主句过去进行时，when从句用一般过去式：表示当一个动作发生时，另一个动作正在进行We were reading when Mr zhang came into the classroom.(强调主句）2.when从句用过去进行时，主句用一般过去时:表示当一个动作正在进行时，另一个动作正在发生。

When were reading , Mr zhang came into the classroom.（强调从句）。