about to, be going to

about to意为“马上就要”；“正要”。

We're about to set out.我们马上就要出发了。

I was about to go to bed when there was a knock at the door.我正要睡觉，忽然听到敲门的声音。

在美国英语里，not about to可以表示“不愿意”。

I'm not about to give him so much.我不愿意给他那么多。

be about to 这个复合助动词相当于be just going(或 ready) to do something 或 on the point of doing something, 含有“急促”的意思，不必加饰soon一类副词；但可以加饰由when连接的时间从句。be going to作“行将”讲时没有这类限制。

例如：

He is about to start.他快要启程了。(毋需加饰soon或者at once)

I was about to say something when your interrupted.我刚要开口说点什么，而你突然插起嘴来。

We're going to spend our holidays in Wales this year.我们今年将去威尔士度假。(可以加饰时间状语this year)