“当某事发生时某人正在做某事”的英语语法是什么？

1、当某人正在做某事，这时发生了什么：
sb. be doing sth when...
例：I was doing eighty when I overtook you.
2、当某人刚做完某事，这时发生了什么：
sb. had just done sth when...
I had just finished my report when someone knocked at the door.
3、当某人正要做某事，这时发生了什么：
sb. be about to do sth when...
One day, I was about to do some shopping when he telephoned me.

be doing表示将来时与be going to 的区别？将来时的问题？

表示一般将来时的六种结构及其用法区别
摘要：表示在将来某个点时间或段时间要发生的动作用一般将来时。现代英语中表示将来信息的结构有六种。这六种结构既有共性又有个性，用于不同的场合，其感情色彩也不同。
一．表示将来的六种结构：
1). S.+shall/will+v
2) S.+shall/will+be+v-ing
3) S.+be+going+to+v
4) S.+be+v-ing
5) S.+be+to+v
6). S.+v
关键词：
一般将来时 结构 感情色彩
正文：
表示在将来某个点时间或段时间要发生的动作用一般将来时。现代英语中表示将来信息的结构有六种。这六种结构既有共性又有个性，用于不同的场合，其感情色彩也不同。
一．表示将来的六种结构：
1). S.+shall/will+v
2) S.+shall/will+be+v-ing
3) S.+be+going+to+v
4) S.+be+v-ing
5) S.+be+to+v
6). S.+v
二．六种结构在用法上的区别;
1) S.+shall/will+v 的用法：
在传统语法中，shall和will都被看作是一般将来时的助动词，shall用于第一称，will用于第二人称。现代英语中趋向于用will。这种结构只表达未来，没有语气和感情色彩。例如：
1. I’ll go to Beijing next week.
2. When shall I see you next?
3. It’ll be Sunday tomorrow.
2) S.+shall/will+be+v-ing 的用法：
这一结构表示将来带有感情色彩，表示礼貌或委婉。因此，这一结构在口语中经常出现。例如：
1. What will you be doing tomorrow?
2. I’ll be waiting for you at the gate.
3). S.+be+going+to+v 的用法：
这一结构经常用于口语，如：
（1）.用于主观意图、打算、目的:
1. We are going to see the film.
2.I’m going to write him a letter.
(2). 用于对客观事物的判断:
1.It’s going to rain this afternoon.
2.It’s going to be a fine day tomorrow.
(3).说明决心：
1. She is going to have a baby.
2. She doesn’t like the meeting ,she is going to arrive late.
(4).用于条件句中表示将来:
1.If you are going to see the film, you’d better come early.
2.You must be hurry if you are going to attend the meeting.
4). S.+be+v-ing 的用法：
这一结构经常用在口语中，表示计划、安排要做的事情。动词常是一些表示位置移动的动作如：come,go,arrive,leave,fly,等等。例如;
1. My aunt is arriving this afternoon.
2. Tom is flying to Beijing tomorrow.
3. She is leaving for Shanghai next week.
5). S.+be+to+v 的用法：
该结构较为正式，可以表示计划要发生的事、命令或禁止或者一种预测或注定。例如:
1. The sports meeting is to take place soon.
2. The angel flied to Maria and said : “You are to have the baby”.
3. You are to do your homework before you go to bed.
4. All these things are to be answered for.
5. Happy days are soon to follow.
6. They are to win the basketball match.
6). S.+v 一般现在时表示将来：
用法（1）.按时间表进行的事情;
1. School begins on September 12th.
2. The train arrives at 7 a.m.
用法 (2). 在时间和条件句中表示将来：
1. I’ll tell him about it as soon as he arrives.
2. The harder you work, the more you’ll get.
用法 (3).在让步状语中表示将来：
1． Whether she comes or not, I’ll go there.
2． No matter what he says , I won’t listen to him.
用法 (4).在某些固定搭配中表示将来:
1. I’ll go to help him the moment he asks me for help.
2. She is going to see me the day she comes.
3. I’ll ring you up directly I hear the news.
用法 (5). 用于hope, see (to it ) suppose, assume, take care, be careful, make sure/certain 的宾语从句中，代替一般将来时：
1.I hope the weather keeps/will keep up.
2.I’ll see (to it ) that nobody troubles/will trouble you .
3.Be careful that you don’t make the same mistakes next time.
除了以上所例举的用法，另外，be about to do 也可以指“马上”或“即刻”要发生的事情，这种结构是书面体，较为正式，一般不用时间状语。例如：
1. The bus is about to go.
2. They are about to leave for Shanghai.
另外，be about to… when…是固定搭配是指：“刚一…，就…”
1. She was about to go out when it rained.
2. I was about to go to bed when there was a knock at the door.
参考文献：
1．《牛津高阶英汉双解词典》
2．《薄冰英语语法》