4. After Columbus discovered America, he returned to Spain.
3. By the time he actually found a job, he had applied to a number of companies.
6. The minister admitted that he had taken bribes from several businesses.
5. My sister had eaten all the biscuits before I came home.
7. They had gone out together for almost 10 years before they decided to tie the knot.
8. She did not realize that she had not packed her present for her aunt.
9. The teacher asked me whether I had stayed up late the night before.
10. He rushed downstairs, only to find that somebody had stolen his bike.
Giải thích:
- Các câu trong đề bài đều là câu hoàn thành trong quá khứ, nên cần sử dụng thì quá khứ hoàn thành (had + V3) để diễn đạt sự việc xảy ra trước một thời điểm khác trong quá khứ.