这里不是过去完成时的问题系,而是属于 it 指代不明的错误.
　　Peter the great totally changed the landscape of St.Petersburg 和 (and it had been) a swamp unusedby previous tsars 是并列句.第一分句中的宾语是 landscape of St.Petersburg,其中landscape 指事物,St.Petersburg 指地点,都可以用代词 it 指代.所以,第二分句的主语 it 属于指代不明.it had been a swamp 既可以理解为 the landscape had been a swamp（这里的地貌曾经是一片沼泽）,也可以理解为 St.Petersburg had been a swamp （圣彼得堡曾经是一片沼泽）.