a dear old lady （who had always been so kind to everybody）, had（，in her youth, ）poisoned every one of her five husbands.

现在看明白没有，主语 a dear old lady, 谓语动词 had poinsed ...

in her youth是插入的状语，所以用逗号隔开。

had一个词，怎么能是插入语呢？提问之前自己先要想一想。