请教一个时态的用法

问题描述

That's about how high an object would have to be orbiting straight up from the equator to constantly remain directly above the exact same spot on the rotating planet Earth."would have to be orbiting"是什么时态？为什么要使用这种时态？

老师答疑

刘老师：

That's about how high an object would have to be orbiting straight up from the equator to constantly remain directly above the exact same spot on the rotating planet Earth.

【答】have to be orbiting：to be orbiting 是不定式的进行式，表示正在进行的动作，或者应该在进行，说明动作的具体和生动。不过在have to之后较少使用，在其他情况下使用较多（如：It seemed to be raining. He pretended to be reading）。

又如：

I'm afraid I'll have to be going. 恐怕我得走了。

Actually, we'll have to be thinking of going. 我们真的想要走了。

And when do we have to be doing the research?  我们什么时候做此研究?

I'm afraid we really have to be pushing along now. 恐怕我们现在真的该走了。