I buy a book for ____(they)

答案是 with.
with用法
I.“
同…一起；跟…一起；和…；与…一同”
be connected with,be friends with,be familiar with,celebrate… with,come with,chat with,compare with,change with,catch up with,communicate with,deal/trade with,discuss with,eat… with salt and butter,go with,go camping with,interview with,keep / get in touch with,live with,have a word with,sign with,spend vacation with,shake hands with,share with,speak with,talk with,work with,together with,etc.
II.“
有；带有；戴着；具有；怀有”
1.　1.Anyone with the right knowledge can give first ai
2.　2.I think the ones with nuts in them are nice
3.　3.She married a man with a lot of mone
4.　4.He was a little man with thick glasse
5.　5.The countryside tre is very beautiful too,with lots of mountains and rivers.
6.　6.He is young,but I never know so young a man with so wise a hea
7.　7.Hank Stram was on vacation travelling through Europe by train with his childre
8.　8.Now they were goingo Bonn with the tickets and passpprts.
9.　9.One day a young lady came to visit Sherlock Holmes with a personal proble
10.　10.I always carry matches with me at such time
11.　11.Nerissa,dressed like a lawyer clerk,arrives with a letter from the fams lawyer whom Portia has visited.
a coutry with a small population,a country with plenty of space and a good climate,a country with sea coasts,films with sound / dialogue,people with interesting jobs,police officers with guns,a space tocket with no rings,water with a depth of less than 60 meters,the one with a beard,a woman with patience and imagination,etc.
III.“
用…方法/工具”
climb up…with ladders,close…with your fingers,write with a pen ( write in ink),etc.
IV.“
用（材料、内容）覆盖；填充”
1
．　 表示“供应；供给；备有”
2
．　 表示在某物表面上有某物,“排列着；陈设；装饰；点缀”
V.“
以…态度/方式；…地”

at a good price-就是固定搭配
如：
Where can I get rechargeable batteries with recharger at a good price?
The shares were bought at a good price.
If they can get it at a good price they'll take it.
left---肯定已经离开了.事情已发生的话就要用过去式,哪怕是一分钟之前已经算是过去.