This is not so good and flexible a course!
I wrote down "ban chuan bi thang nhung ban muon dung lai" and it didn't work at all!
Most likely because chuẩn bị is prepare and sắp is about to
Accepts nhưng, too. I really wonder why using 'mà' and 'vậy mà' for "but" is punished so ruthlessly elsewhere in the course but it taught here. In talking with Vietnamese friends, I often use 'mà' for nhưng and they don't correct me as wrong.