请问这句英语如下

这是个虚拟语气，前面一句是从句，后面是主句。但是从句省略了连词if,所以将had提前，进行倒装
也可以说成：
If they had known their neighbour was a drummer, they wouldn't have moved into the building.

如果他们早就知道邻居是个鼓手的话，他们就不会住进这栋大楼了。
这是倒装句，所以had放在句首了。

虚拟语气把HAD提前，相当于If they had known...，这是一种语法现象。

倒装句，正确语序是they had known的。