想让他答应这样的要求恐怕不大可能。

[误] I'm afraid it is impossible for him to agree to such a request.

[正] I'm afreaid it is unlikely for him to agree to such a request.

注：impossible 表示“完全不可能”，所以与原文有出入。在英语中，probable 表示的可能性最大，其次是
possible，再次是likely。而常用的句式为it is probable/possible/likely for sb. to do sth.，或 sb. be likely to do sth.。