1) 句首为否定或半否定的词语，如no, not, never, seldom, little, hardly, at no time, in no way, not until; 等。
Never have I seen such a performance.
Nowhere will you find the answer to this question.
Not until the child fell asleep did the mother leave the room.
I have never seen such a performance.
The mother didn't leave the room until the child fell asleep.
1) Why can't I smoke here?
At no time___ in the meeting-room
A. is smoking permitted B. smoking is permitted
C. smoking is it permitted D. does smoking permit
答案A. 这是一个倒装问题。当否定词语置于句首以表示强调时，其句中的主谓须用倒装结构。 这些否定词包括no, little, hardly, seldom, never, not only, not until等。本题的正常语序是 Smoking is permitted in the meeting-room at no time.
2) Not until the early years of the 19th century ___ what heat is.
A. man did know B. man know C. didn't man know D. did man know
答案D. 看到Not until;的句型，我们知道为一倒装句，答案在C，D 中选一个。
改写为正常语序为，Man did not know what heat is until the early years of the 19th. 现在将not提前，后面就不能再用否定了,否则意思就变了。