1. What it was to become was uncertain until …

他昨天直到七点才离开学校.

He didn’t leave school until seven o’clock.

他们到了中国才能看了长城.

They couldn’t see the Great Wall until they came to China.

It was not until they came to China that they could see the Great Wall.

Not…until…直到…才…

It is/was not until… that… 强调句

It was not until seven o’clock that he left school.