介词until（till）与by的用法区别

用作介词的until（包括till）与by，均可以表示“在某时之前并不晚于某时的时间”
1. util/till通常只与表示一段时间的持续动词连用，如（stay，wait etc.），如：
She stayed until noon 她呆到中午才走
It may last till Friday 这可能得持续到星期五
I remain in london till May我在伦敦呆到五月
We talked until midnight and then seprated 我们一直谈到深夜才分手
It was his intent to leave at 6:30,but he stayed until 8:00他本打算六点半离开，但是他一直呆到8:00
We are counting down the days until the end of exams我们倒数着日子，直到考试结束
注：若所有动词只表示一时点（如finish，leave），则只能在否定句中用until/till.如：
You mustn't open this parcle until Christmas Day.圣诞节前你绝不能打开这个包裹
We got home at 6 o/clock, but the other didn't get back until 10。我们六点到家，而另外几个10点才到
He promised to be here at four o'clock but didn't appear until six.他答应四点来，结果六点才到
I arrive  at the hospital at eight o'clock,but didn't go on duty until nine我八点就到医院了，但是九点才去值班
Women were not given the franchise in Britain until the twentieth century.英国父女到而是世纪才获得选举权
2. 介词by通常只与表时间点的动词连用。如：
She'll have retired bu then.她到那时就要退休了
We had hoped to finish by four o'clock我们原希望四点钟前完成
I'll have left by Monday.(=any time befor and not later than)到星期一我就已离开了（=在星期一前不晚于星期一的任何时间）
We're in big trobule if we don't come up with the money by 6 o'clock如果我们六点钟之前拿不出钱来，麻烦就大了
注：有些表示状态的动词在某些特殊搭配或语境中可以表示时点，此时也可与by连用，如：
As a rule i'm home by six我通常六点钟到家（=As a rule I get home by six）不能理解为‘通常六点钟前我呆在家里’
She worked hard so that everything would be ready by 6 o'clock此句应该理解为“她拼命的干以便六点时把一切都准备好”（相当于She worked hard so that everything would get ready by 6 o'clock）,不能理解为“他拼命干以便能到六点钟时一切都一直是准备好的”
其他例句如：
We shall be back by 6 o'clock.我们六点钟回来
The children must be back by 4 o'clock. 孩子们必须四点钟前回来
I drove home to him that he must be here by ten.我跟他说的清清楚楚：他必须十点钟以前到这

3. 当与表时点的动词连用且用于否定句时，两者都可以用。如：
I won't have left by Monday.(=I'll still be here on Monday)我星期一前不会离开的。（=我星期一还会在这
）
Might I enquire why you have not mentioned this until now?请问你为什么直到现在才提及此事？