新概念3第1课、第三课的疑问

题目

新概念3第1课、第三课的疑问
Lesson1
Paw prints were seen in anumber of places and puma fur was found clinging to bushes.Several people complained of 'cat-like noises' at night and abusinessman on a fishing trip saw the puma up a tree.The experts were nowfully convinced that the animal was a puma,but where had it come from?(疑问：这里为什么不用where did it come from?必须用had吗?有什么区别?) As no pumas had beenreported missing from any zoo in the country,this one must have been in thepossession of a private collector and somehow managed to escape.
Lesson3
Some time ago,andinteresting discovery was made by archaeologists on the Aegean island of Kea.An American team explored a templewhich stands in an ancient city on the promontory of Ayia Irini.The city atone time must have been prosperous,for it enjoyed a high level ofcivilization.Houses -- often three storeys high(这里often three storeys 是指房子一般有3层,还是指房子很高,一层楼像平常的三层楼一样高?) -- were built of stone.

答案

第一课那是个过去完成时,描述在过去的过去发生的事情.美洲豹来在前,人们发现它在后,所以而人们发现它对我们读者来说又是过去的时间,所以这就算是一个过去的过去.而且,完成时强调的是影响,一般过去式强调的仅仅是过去发生过某个事情.这里很显然强调对当时的影响.第三课指的是房子有三层.