1. be disappointed 后接介词，情况比较，大致区别如下：
(1) 表示对某人感到失望，通常用介词 with, in。如：
I’m disappointed in you! How could you have lied likethat? 你让我失望了，你怎么能像那样说谎呢?
My parents will be disappointed in [with] me if I failin the exam. 要是我考试不及格，我父母会对我感到失望的。
(2) 表示对某一行为(doing sth)感到失望，通常用介词 at, about等。如：
We are all disappointed at [about] his failure. 我们都为他的失败而感到失望。zn
He was really disappointed about [at] not being able togo. 他未能去成实在感到失望。
(3) 表示对某一事实或情况感到失望，一般用介词 with, at, in 等。如：
He’s disappointed at [in] the result. 他对结果感到失望(from www.yywords.com)。
He must be very disappointed with [at, about] your examresults. 他对你的考试成绩一定非常失望。
(4) 表示因得不到某东西或实现不了某愿望而失望，通常用介词of。如：
I was disappointed of the car. 没等到车子我感到很失望。
He was disappointed of his ambition. 他因实现不了抱负而沮丧。
2. be disappointed 后接不定式。如：
He was disappointed to hear the news. 听到这消息他很失望。
I was disappointed not to be chosen. 我没被选中，感到很沮丧。
3. be disappointed 后接 that 从句。如(from www.yywords.com)：
We were disappointed that you couldn’t come. 你不能来我们感到很失望。
I’m rather disappointed that he should get the post. 他竟会得到这个职位，我感到很失望。
4. 比较 disappointing & disappointed：
(1) disappointing 表示“令人失望的”，通常用于事物给人的感觉。如：
What disappointing weather! 天气真叫人失望!
Your work is really disappointing. 你的工作真是令人失望。
若用于“人”，则表示该人令别人失望。比较：
The boy was very disappointing. 这男孩很令人失望。
(2) disappointed 表示“失望的”、“感到失望的”，通常用以说明“人”自己的感觉。如：
She looks disappointed. 她看起来很失望。
I am very disappointed with my new bike. 我对我的新自行车感到很失望。
若用于修饰 voice, look, expression, appearance, manner等名词，则表示与之相关名词的逻辑主语感到失望。比较：
We noticed a disappointing look on his face. 我们注意到他脸上一种令人失望的表情。(“他”并不失望，但见了他表情的人会失望)
We noticed a disappointed look on his face. 我们注意到他脸上有一种失望的表情。(感到失望的是“他”，不是别人)