On FCE we have the following exercise:
- Complete the second sentence so that it has a similar meaning to the first sentence, using the word given. Do not change the word given. You must use between two and five words, including the word given.
d. She had to finish her homework before she went out. (until) She had to stay in ________________ her homework.
I'm having a hard time to grasp the two events rule on this one! "She had to stay in until she finished her homework." Sounds perfectly fine to me! Why do I end up with "She had to stay in until she HAD finished her homework."