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".不过,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 .
再问： 是不是某种程度上可以说until具有否定含义呢
