課本上的原句是這樣的：

In the nineteenth century, a novelist would bring his story to a conclusion by presenting his readers with a series of coincidences—most of them wildly improbable.

其中的most of them wildly improbable為獨立主格結構，在句中起補充說明的作用。全句意為：在19世紀，小說家常在小說結尾處給讀者準備一系列的巧合——大部分都是極不可能的。

如果按你的意思加上are / were，也可以，那就不是獨立主格結構了，此時同時還應加上連詞，使之成為并列句，即改為：

In the nineteenth century, a novelist would bring his story to a conclusion by presenting his readers with a series of coincidences, but [and] most of them are [were] wildly improbable.