Only when he arrived home _____ that he had left his key in the office.

Once          a difficult and even dangerous place , Vietnam is now a friendly detestation that welcomes visitors from all over the world        .

Not until he failed in the last examination _________ he should have spent more time on study.

  – Sir, we have failed to work out this problem. What is exactly the right way?--- Look. Only in this way ________ , but you boys _____ a wrong way.

Not until he retired from teaching three years ago       having a holiday abroad.