I think it's impossible for him to finish it in ten minutes.

I think it's impossible for him to finish it in ten minutes,isn't it?
含有think, believe, suppose, imagine, expect等动词后接宾语从句构成的主从复合句在构成反意疑问句时,视情况不同有两种不同的构成方式. 　　
(1)当主句的主语为第一人称时,其后的简短问句应与从句相一致.例如： 　　
I expect our English teacher will be back this weekend, won't she/he? 　
We suppose you have finished the project, haven't you? 　　
值得注意的是,当这些动词后接的宾语从句的否定转移到主句时,其仍属否定句,故其后的简短问句应用肯定式,而非否定式.例如： 　　
I don't believe that he can translate this book, can he? 　　
We don't imagine the twins have arrived, have they? 　　
此类句子的回答同"前否后肯"型反意疑问句一样,如上述后一个句子,若双胞胎已经到了,则回答为"Yes, they have."；若尚未到达,使用"No, they haven't.". 　　
(2)当主句的主语为第二、三人称时,其后的简短问句则应与主句相一致（此时,否定只看主句,与从句无关）.例如： 　　
Your sister supposes she needs no help, doesn't she? 　
You thought they could have completed the project, didn't you? 　　
They don't believe she's an engineer, do they? 　　
She doesn't expect that we are coming so soon, does she? 　　
(3)但如果主句的时态是过去时等等,疑问句应和主句的人称时态保持一致.