He stood up as if (he were) to say something

e to do 打算做什么
省略he were,是因为as if后面成分的逻辑主语也是主句的主语he
例如She looked anxiously at her parents as if entreating forgiveness.
她忧虑地看著她的双亲,好像在请求宽恕.

为什么he were 要省略？.这是语法习惯，语法省略句型中会学到。为什么要用were to 形式，而不是might be 。带入were to进去翻译为他站起来，好像有话要说。带入might be为他站起来，好像可能说些什么。

be to do something
准备做某事，固定用法。
省略he were 也是固定用法。
as if to do .....