语法题,求英语强人解释下!语法很烂

题目

语法题,求英语强人解释下!语法很烂
We___ finished our assignment when the teacher came to the door.
答案是had just had,我想问问为什么是两个had?为什么just要放在中间?
,Catherinehas never dreamed of ____ abroad.
答案是being sent,知道dream of 后接doing,我想问问为什么是表被动语态,ing不是主动吗?为什么又可以表被动?
It was the training at college——made him such a good writer.
答案是that,那是who可以吗?为什么?强调句只可以用that吗?
还有从句中“,”后只能接which吗?what那些都不行吗?
分剩不多了所以给分不多,不好意思啦,求解答QAQ

答案