我想问一下On 的用法 在什么情况下是什么意思

我想问一下On 的用法 在什么情况下是什么意思
下面两个句子里 On起什么作用 他为什么会是那个意思
1.the students union held an interesting debate (on) capital punishment yesterday.
2.they will have finished work (on) the new stadium
这两个On意思好象都不一样 我看到它们就是不能理解 所以我没办法记住.

1 = about
2 说不好 大概嘛 也是关于什么的意思
关于On的用法：
(1) 作介词：
1. 表示位置：在……上
Mary’s coat is on the desk .
They’re going to work on a farm .
There are many apples on the trees .
I have them on this piece of paper .
There was a big smile on her face .
Jim and Li Lei meet on the road outside Li Lei’s house .
Which channel is it on ?
2. 表示时间：在……（天）
Everyone eats mooncakes on Mid-Autumn Day .
They arrived early on a Tuesday morning .
I eat mooncakes on that day .
I was born on February 18 , 1981 .
Either Tuesday or Wednesday is OK . I’ll be free on these days .
3. 关于……
In the library she has got books on many different subjects .
(2) 作副词,与动词连用,构成短语：
1. come on跟我来,加油
“Come on !”they shouted .
2. get on上（车）
Let’s get on the bus .
3. get on with（sb. ）与（某人相处）
Usually we get on very well with each other .
4. go on继续
Autumn starts in August , and goes on to October .
Go on until you reach the end .
5. hold on（for a moment）别挂断电话,等一等
A : Hello ! Could I speak to the headmaster , please ?
B : Hold on for a moment , please . I’m sorry , he isn’t here right
now . May I help you ?
6. put on穿上,戴上,上演
Put it on , please Lily .
Don’t be late——oh , and put on your old clothes .
7. try on试穿（衣服）,试试看
Shall I ask the shopkeeper if I can try it on ?
8. turn on开,旋开（电灯,无线电等）
I turned on the radio .
Turn it on .
9. pass on传递
They both passed on their sticks at the same time .
10. go on with（sth. ）继续做……
They went on with their work .
11. walk on继续走
Walk on and turn left .
(3) A. 作介词,与动词连用,构成短语：
1. go on继续
He quickly got up and went on running .
2. spend … on在……花费
I like writing to my penfriend , but I can’t spend much time on it .
3. keep on继续
I kept on thinking about the match in the afternoon .
4. knock on敲
The boy is knocking on the door .
5. work on致力于
He works on the Great Green Wall with many other people .
Please tell Mr. Hu that I am working hard on my Chinese .
6. live on靠……为生
They lived on potatoes .
7. call on访问,拜访
I called on you .
B. 作介词,与名词连用,构成短语：
1. on foot走路,步行
I see you’re on foot today .
2. on the / one’s way to在途中,在路上
On her way home she bought a new pen at a shop .
3. on time 准时；不早不晚
You must always return your library book on time.
4. on duty值日
Who’s on duty today .
5. on show展览；陈列
They saw many old things on show in glass-topped tables .
6. on（the）earth在地球上
Two thousand years ago , there were only 250 million people on the
earth .
7. on the left/ right在左（右）边
Walk along this road and take the fourth turning on left .
8. on the radio通过收音机,以无线电的方式
She said that she heard a very interesting programme on the radio .
9. on TV通过电视,以电视的形式
We can watch a football match on TV .
10. on top of在……上面（顶部）
Can you see much grass on top of the rock ?
11. on the team在队里
Neither of them is on the team .
12. on a visit在访问
Xia Yi is on a visit to England .
13. on fire着火
The building is on fire .
14. on holiday休假,度假
You and your family are away on holiday .
15. on sale出售
The meat is on sale in the market.
(4) 其他
1. and so on等等
There are different kinds of vegetables in the shop , such as
tomatoes , potatoes , carrots , peas , cabbages and so on .
2. on and on继续,不断
They talked on and on very happily .
3. from now on从此以后,今后
Please be more careful from now on .
4. later on过后,以后
The radio says the rain will stop later on