once的用法

Question

Once you have made a promise,you must keep it.这里once后面为什么要用完成时态，可不可以用其他的时态
如果是要表示将来的情况或者是虚拟的呢，例如这个句子Once he makes up his mind to do something no one can hold him back.

Answer 1

once是引导从句，主要是看主句是表将来、现在。两个句子都是一样的意思。once其实是很简单的用法，这里你主要是看从句的谓语表示的意味。
have done是指从过去到现在甚至到未来的影响作用，而第二句用一般现在时，主要是表现在的意思。

Answer 2

问题补充：

如果是要表示将来的情况或者是虚拟的呢，例如这个句子Once he makes up his mind to do something no one can hold him back.

这个是条件句：主将从现：

一旦你许了诺，就要遵守。

因为对现在有影响，而且发生在过去，所以要用完成时。本回答被提问者和网友采纳

Answer 3

1.(长期或永久)持有,保有
I kept it all the time to remind me of you.
我一直保存着它,以此唤起我对你的记忆。
2.(一时)拥有;保管
Please keep the watch for me while I go swimming.
我去游泳,请代我保管这表。
3.存放,保留,保存
We keep food in a refrigerator.
我们把食物存放在冰箱里。
4.使...保持在(某一状态)[O][O4][O7][O8][O9]
Those toys kept the children amused.
那些玩具使孩子们玩得很开心。
She kept the child quiet.
她让孩子别作声。
5.整理,料理
Mother keeps the house.
母亲料理家务。
6.履行;遵守;保守
Everyone must keep the law.
每个人都必须守法。
7.阻止,妨碍;控制住[(+from)]
Robbie couldn't keep the child from yelling.
罗比没法使孩子不大叫大嚷。
8.记(日记、帐等)
Mary keeps a diary.
玛丽天天记日记。
9.抚养;饲养
He needs more money to keep his wife and children.
他需要更多的钱来抚养妻子儿女。
10.雇用
She keeps a maid.
她雇一个女佣人。
11.经营,开设(商店等)
They kept a small hotel in the town.
他们在城里开了一家小旅店。
12.守卫;看守[(+from)]
He has kept goal for our team.
他一直是我们队的守门员。
13.庆祝;守(宗教节日等)
They keep Christmas as well as Spring Festival in their house.
他们家既过春节,也过圣诞节。

Answer 4