《剑桥英语语法》 page 171写道:
【ib】 #The sun is setting at five tomorrow.
The progressive is restricted to cases where human agency or intention is involved – hence the anomaly of examples like [ib].
作者觉得进行体表示以后仅限于涉及人有哪些用途或愿望的状况下,因此【Ib】例句是错的。按此说法,It's raining soon.和 The sun is setting soon. 是不是都是错的?
It's raining soon.错句。It's raining 只有在下雨时才如此用。但可以说It'll be raining soon. It will rain soon. 或It's going to rain.
The sun is setting soon. 正确。这是客观状况描述,不是是human agency. 和《剑桥语法》的例句不同,由于没具体时间at five.