They were playing by the door a short time ago.(改为否定形式的疑问句）

were not they playing by the door a short time ago一般疑问句的方法是 把BE 动词或 助动词提到 主语的前面。再变成否定形式的话 就在 BE 动词或 助动词的后面加上 not 就行了。