until的用法
until（或till）在句子中用作介词或连词，其含义和since有类似之处，只是since强调的是某一段的时间的起点（“从……”），而until表示的是一段时间的终点（“到……”）。
一．由until （或till）构成的介词短语或引起的从句在句中作状语时，如果是肯定句，句子的谓语动词或主句中的谓语动词一般是延续性的；在否定句或含有否定意义的句子中，句子的谓语动词或主句中的谓语动词，既可是非延续性的，也可以是延续性的。肯定句侧重动作延续的时间，否定句说明动作开始的时间。
例如：
They thought their bodies would be well kept in these until they would come back to life.
He and the other doctors did not leave until the operation was over.
They are not going back to work until they get it.
He doesn’t sit down until the other person takes his seat.
And then they went on with their work at the empty loons until late into the night.
No more work was done on it until the People’s Republic of China was founded.
二．对含有“not … until …”结构的句子进行强调时，须用it is (was )not until…that…。
例如：
It was not until she took off her dark glasses that I realized she was a famous film star.
It was not until I saw Margaret the next morning, however, that I felt happy.
三．如果把“it is (was )… that ….”这个强调结构的框架去掉，句子就须用倒装语序。若是一个复合句时，要倒装主句中的主谓部分。
例如：
Not until the early years of the 19 th century ____ what heat is.
A. man did know B. man knew C. didn’t man know D. did man know
答案是 D
Not until I began to work _____ how much time I had wasted.
A. didn’t I realize B. did I realize C. I didn’t realize D. I realized
答案是B