1.He didn't pass the exam 
-> He 
2. They felt unsafe until they had crossed the border .
-> Not until 
3. She said nothing when she left the room . 
->She left the room 
4. I missed the train 
-> By the time 
5.The flight was cancelled because of the heavy rain 
-> But 
6. Unless you study hard , you won't pass the final test . 
-> Study hard 
7. She rang as soon as they locked the door. 
-> No sooner 
8. They didn't wait for the specialist . 
-> If 
9. Going out would be better than staying at home 
-> I'd 
10.There is nothing i can't do for you 
-> I am willing