doing
doing 与to do的区别？
与to do的区别？
时间、功能、含义不同。
1、to do 和 doing 是动词的两个不同的发展阶段，to do 表示还未发生，而doing表示正在发生，所以凡是需要表示还未发生的那么就接动词 to do， 凡是已经发生，或者正在发生的，那么就用动词doing。
2、学习语法的目的，是为了可以更好地进行听说读写等语言实践活动。也就是说，通过对语法规则规律的掌握，减少在语言实践练习中的盲目性，提高语言的准确性。
3、关于to do的短语，allow sb to do sth允许做某事;ask for sth 请求某物;ask sb (not) to do sth请求某人(不)做某事;afford to do sth 有能力做某事;agree to do sth同意做某事;be afraid of doing sth害怕做某事。
be doing 可以作系动词吗？
be doing 不可以作系动词，详解如下。
系动词，亦称连系动词（Link Verb），系动词是用来帮助说明主语的动词。作为系动词，系动词本身有词义，但不能单独用作谓语，后边必须跟表语，亦称补语，构成系表结构说明主语的状况、性质、特征等情况。英语系动词的功能主要是把表语（名词、形容词、某些副词、非谓词、介词短语、从句）和系动词的主语联系在一起，说明主语的属性、特征或状态。
when doing和upon doing和once doing的区别？
when doing sth 是省略形式的状语从句，分词的逻辑主语是主句的主语，表示“当……时”，有时候也可表“一……就……”的意义。
如：
When staying at home, I often play chess with my grandpa. 在家时，我常和爷爷下棋。(When staying at home When I stay at home)
When seeing her mother, the little girl dropped her toy and rushed to her. 一看到妈妈，小姑娘就丢下玩具向她跑去。(When seeing her mother When / As soon as she saw her mother On seeing her mother)
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 cant return underwears.
You cant return underwears once youve tried them on.
Once youve tried them on, you cant 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。