[译] 老师对那个粗心的学生感到失望。
[误] The teacher was disappointed at his careless student.

[正] The teacher was disappointed in his careless student.
[注] 表示“对某人感到失望”，应该说 be disappointed in sb., 其中介词用 in. 而 be disappointed at 用来表示“对某事或做某事而感到失望”。如: He was disappointed at not finding his lost wallet. 他因没有找到他丢失的皮夹而感到失望。