=>
1. was playing/were reading
-> QKTD while QKTD (hai hành động đồng thời đang diễn ra trong quá khứ)
2. has just met
-> have/has just V3/ed
3. has he learned
-> how long + HTHT : bao lâu
4. We haven't seen her since we left for school.
-> chuyển QKĐ sang HTHT, S have/has (not) V3/ed, since + mốc thời gian
5. It is not difficult to learn Maths.
-> It's (not) adj to V
6. carelessly
-> drive là động từ -> cần trạng từ
7. She asked me why I had a day off the following day.
-> câu tường thuật dạng WH questions : asked sb + từ để hỏi + S V(lùi thì, HTĐ -> QKĐ)
8. Rice is grown in tropical countries.
-> bị động HTĐ is/am/are V3/ed
9. Mary spent two hours travelling to the countryside last week.
-> spend time Ving ~ take sb time to V
19. different 
-> be different from : khác