$17.$ Who will captain the team if Nick isn't available $?$

`-` Who dùng để hỏi người

$18.$ When did you see him $?$

`-` When dùng để hỏi về thời gian

$19.$

`⇒` What do you often listen to classical music for $?$

`⇒` Why do you often listen to classical music $?$

`-` What for `:` Để làm gì

`-` Why dùng để hỏi nguyên nhân

$20.$ How long did the Bach concert last $?$

`-` How long `:` Bao lâu

$21.$ Who did you go on a holiday with last summer $?$

`-` Who dùng để hỏi người

$22.$ How many people were there at the concert last night $?$

`-` How many `+` N ( Đếm được số nhiều ) `:` Bao nhiêu

=>
17. Who will captain the team if Nick isn't available?
-> who (hỏi người : ai?)
18. When did you see him?
-> when : khi nào
19. What do you often listen to classical music for?/ Why do you often listen to classical music?
-> what for : để làm gì
-> why : tại sao
20. How long did the Bach concert last?
-> how long : bao lâu
21. Who did you go on a holiday with last summer?
-> who (hỏi người : ai?)
22. How many people were there at the concert last night?
-> how many + N đếm được số nhiều : bao nhiêu