67. I didn't realize my mistake until I entered the office.

点睛　

not...until句型使用广泛，表示“（不）……直到”。其中，not可以省略。如：
Wait until tomorrow. 等到明天。

考点归纳　

与not...until句型类似的是till，二者用法的区别比较微妙，需要特别注意：
当主句是否定句时，意思是“直到（某时）（某动作）才（发生）”，这时候常会使用not until的结构；如果将not until结构放在句首，那么主句要用倒装形式。如：
1) He didn't go to sleep until 12 last night. 他昨晚直到12点钟才睡觉。
2) Not until we pointed out their fault to them did they realize it. 直到我们指出他们的错误，他们才意识到。
当主句是肯定句时，意思是“直到（某时某动作停止了）”，这时候常会使用till。如：
1) The students made much noise till the teacher came into the classroom. 直到老师走进教室，学生们才停止了吵闹。
2) Don't open it till your birthday. 等到你生日那天再打开。
另外，until可以放在句首，而till则不行。如：
1) Until they used up all their money, the young couple were very happy. (√)
2) Till they used up all their money, the young couple were very happy. (×)