1. While they were travelling to Ha Noi, they met their old friends.
- During + N/NP... = While + S + V...
2. The film we saw last night was very interesting.
- S + V + (a/an) + adj + N... = The + N + S + V + be + adj.
3. They make me come home at 9 o'clock every night.
- make sb V-inf: bắt ai làm gì.
4. The air in most classrooms in this school contains a lot of harmful gases, which is very worrying as many young children are studying here.
- Từ "This" trong câu cho sẵn được thay thế bằng đại từ quan hệ "which" ở câu viết lại.