9. How long have they been here ? 

- I don't know exactly. Maybe, they have been here since 7 o'clock

Cấu trúc : How long + thì hiện tại hoàn thành( S + have/has + P2) 

10. He came into the study, stared at me and left without saying a word. 

Những điều này đã xảy ra nên chia thì quá khứ, S + P1 

9 have they been ( how long + have / has + S + VPII )  - don't know( HTĐ phủ định : S + don't / doesn't + V nguyên mẫu ) - have been ( thì HTHT + since : S + have / has + VPII + since + thì QKĐ , mốc thời gian ) 

10 came - stared - left

thì QKĐ ; s + v-ed / v2