Her words were amazing then.这句话是什么结构?

Her words were amazing then.这句话是什么结构?
语法书上说是过去进行时.但我认为是系表结构.我的分析：were是连系动词be的一般过去式,amazing是现在分词做形容词.由于分词可以有自己的状语,所以then是amazing的状语.
怎么回事?
试比较：The news is exciting.(这里是系表结构)