1、on /upon doing sth
刚一做某事（就）
结构：介词 动名词
功能：相当于as soon as引导的时间状语从句。
例如：
On arriving home I discovered they had gone.
我一到家就发现他们已经离开了。
= As soon as I arrived home，I discovered they had gone.
= On my arrival home I discovered they had gone. ( on/upon doing必须注意，后面句子主语就是前面doing 动作的发出者。)
On arriving at the village,these college students went to help the farmers with their work.
=As soon as they arrived at the village,these college students went to help the farmers with their work. 这些大学生一到村里就去帮助农民工作。
= On their arrival at the village,these college students went to help the farmers with their work. ( on/upon doing必须注意，后面句子主语就是前面doing 动作的发出者。)
Upon going out of the room,Jack heard his cellphone ringing.
=As soon as he went out of the room,Jack heard his cellphone ringing.( on/upon doing必须注意，后面句子主语就是前面doing 动作的发出者。)
杰克一走出房间，就听到他的手机响了。
2、once doing
一旦做某事
结构：连词once 加动名词，可以看作是once 条件状语从句的省略
功能：相当于once 引导的条件状语从句
例如：
Once trying them on, You can't return underwears.=
You can't return underwears once you've tried them on.
=Once you've tried them on, you can't return underwears.
内衣一旦试穿后就不能退。(once doing sth，必须注意，后面句子主语就是前面doing 动作的发出者)
Once making a promise,you should try your best to fulfil it.
=Once you have made a promise,you should try your best to fulfil it.
一旦做出承诺，你就应该尽力履行。(once doing sth，必须注意，后面句子主语就是前面doing 动作的发出者)
特别注意：后面句子主语与once 后面的动词动作是被动关系时，经常使用过去分词。
例如：
Once bitten,twice shy. 一朝被蛇咬，十年怕草绳。
Once created, LUNs need to be assigned to hosts.
=
Once LUNs are created, they need to be assigned to hosts.
LUN一旦创建，就必须被分配到主机。
