看到如此一个强调句：
It is you that is to blame.
非常明显，这里强调的 you 是作主语吧！为何后面的动词用单数 is 而不是复数 are 呢？
10) 在 it 引导的强调句中，who 或 that 后的谓语动词在人称和数上应与前面被强调的名词或代词在人称和数上维持一致。比如：It is the child that makes such a noise. 是那个孩子如此吵闹。/ It is the people who are really powerful. 真的强大的是人民。/ It was they who were wrong. 错的是他们。 / It is I who am responsible for the organization. 这个组织由我来负责。
但在非正式的文体中，即便who 之前是 I, me, you 等，从句也可用动词 is ；即便被强调的是主语，也可用宾格形式。比如：It is not I who is angry. 生气的不是我。/ It is you who is angry. 生气的是你。/ Its me whos to blame. 是我该受责备。
（以上摘自《薄冰英语惯使用方法词典》p.436）
