`1`. His book, which was published last year, became the best seller.

- "which" là đại từ quan hệ thay cho vật là chủ ngữ hoặc túc từ

- Do có tính từ sở hữu "his" nên dùng mệnh đề quan hệ không xác định (có dấu phẩy)

`2`. He used to spend his free time in the library.

- used to + V nguyên mẫu: đã từng làm việc gì trong quá khứ 

`3`. Going out alone at night is not safe.

- It + be + adj + (for SO) + to V = N/Ving + be + adj

`4`. Although it was raining hard, we went to Mary's birthday party in time.

- Although + S + V: mặc dù - although và but không đi chung câu

`5`. They haven't written to each other for two months.

- S + have/has + not + Vpp + for + khoảng thời gian

`6`. Nam asked Nga if she were having a wonderful time there.

- Câu trần thuật yes, no question: S1 + asked + O + if + S2 + V lùi thì

- you `->` she, here `->` there, lùi hiện tại tiếp diễn `->` quá khứ tiếp diễn

1. His book which was published last year became a best seller (dùng mệnh đề Quan hệ)

2. He used to spend his free time in the library ( Cấu trúc Used to+V(bare))

3. Going out alone at night is not safe

4. Although it was raining hard, we went to Mary's birthday on time

5. They haven't written to each other for two months

6. Nam asked Nga if she was having a wonderful time there ( dùng câu gián tiếp)