be (just) doing sth. ...when...
用于表示当某人或某物正在或正要做某事，就在这时突然又发生了另一件事。
Mary was doing her homework when her mother came in.
当她妈妈走进来的时候玛丽正在做家庭作业。
be (just) about to do sth. ... when ...
表示“正要……这时(突然)……”。when在此为并列连词，意为“就在这时(突然)”
He was about to go out when the telephone rang.
他正要出去，电话铃响了。
We were about to start when it began to rain.
我们刚要出发，天就开始下雨了。
just 即将；刚要（与进行时态动词以及about to, going to等连用）
I’m just coming. 我就来。
They were just about to leave when there was a knock on the door.
他们刚要离开，响起来敲门声。
be on the point of doing sth. ...when...
表示“正要做……，就在这时……”
on the point of (doing) sth. 即将…之时，正要…的时候
He was on the point of watching a movie when there was a power failure.
他刚要看电影，这时突然停电了。
As we were on the point of setting out, a light rain began to fall.
当我们就要出发的时候，外面下起了小雨。
be in the act of doing sth. ...when...
in the act of doing sth 正在做…
When we arrived, her mother was just in the act of starting to make dinner.
我们到的时候她妈妈正要开始做晚饭。
had (just) done sth. ... when ...
I had just gone to bed after a very hard day when the phone rang.
在劳累了一天之后我刚刚就寝，电话铃就响了。
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