确实在很多场合可以交替使用，但是它们是有分别的。till / until在表达方式和意义上的特殊性：until是till的强调形式，但是它们表达的意义是相同的，都表示"直到某时"。
当主句是否定句时，它引出的意思是"直到（某时）（某动作）才（发生）" ，这时候常会出现"not until …"的结构，如果将 "not until …"的结构放在句首，那么主句要写成倒装句。例如：
He didn't go to sleep until 12 last night .
他昨晚直到十二点钟才睡觉。
They didn't realize their fault till we pointed it out to them .
直到我们向他们指出了他们的错误，他们才意识到。
Not until we pointed out their fault to them did they realize it .
直到我们向他们指出了他们的错误，他们才意识到。
但是，当主句是肯定句时，它引出的意思是"直到（某时某动作停止了）" 。例如：
The students made much noise till the teacher came into the classroom .
直到老师走进教室学生们才停止了大声喧哗。
The young couple were very happy until they used up all their money .
那对年青夫妇直到花光了所有的钱才沮丧起来。
另外，until 可以放在句首而till则不行。我们可以说：Until they used up all their money, the young couple were very happy .我们却不可以说：Till they used up all their money, the young couple were very happy .
此两个连词意义相同。肯定形式表示的意思是"做某事直至某时"，动词必须是延续性的。否定形式表达的意思是"直至某时才做某事"。动词为延续性或非延续性都可 以。 正确使用这两个连词的关键之一就在于判断句中的动词该用肯定式还是否定式。
肯定句：
I slept until midnight. 我一直睡到半夜时醒了。
Wait till I call you. 等着我叫你。
（在肯定句中可用before代替 Let's get in the wheat before the sun sets.）
否定句：
She didn't arrive until 6 o'clock.
她直到6点才到。
Don't get off the bus until it has stopped.
公共汽车停稳后再下车。
I didn't manage to do it until you had explained how. 直到你教我后，我才会做。
1)Until可用于句首，而till通常不用于句首。
Until you told me, I had heard nothing of what happened.
直到你告诉我以前，出了什么事我一点也不知道。
2)Until when 疑问句中，until要放在句首。
---Until when are you staying? 你呆到什么时候？
--- Until next Monday. 呆到下周一。
注意：否定句可用另外两种句式表示。
(1）Not until …在句首，主句用倒装。
Not until the early years of the 19th century did man know what heat is.
直到19 世纪初，人类才知道热能是什么。
Not until I began to work did I realize how much time I had wasted.
直到我开始工作，我才认识到了我已蹉跎了几多岁月。
(2) It is not until… that…
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