Question

3. Hardly        on the desk when the door burst open (put)

他一把电话放下，门就突然开了。

Answer 1

3. had he put the receiver/telephone down hardly... when...是固定句型。主句要求用' 过去完成时，从句用一般过去时，表示"一…… 就……"。且主句中hardly位于句首时要用部分倒装结构。