英语翻译主要是能说明为什么要这样写?就是每个语法和句子结构.比如说为什么要用disappointed,而不用disappoint.

I am so dispointed on you!

You let me down.
或是
You make me so disappointed!
you made me disappointed very much!
就没那种失望的语气了！

You for allowing me to be disappointed

1楼解释的很好

You let me down.
let sb.down 让某人失望
至于LZ那个disappointed和disappointing 区别如下:
当主语为人时 disappoint 用 disappointed.
当主语为物时 disappoint 用 disappointing.
当修释词为adv.(副词)时disappoint(至于为什么adv.修释v.(动词)原因就不解了 国际规定我也没法)

you made me disappointed very much!
disappointed 在这里是形容词 失望的
而disappoint 是动词 当然你也可以说
you disappoint me so much
那这里的disappointed 就是动词的过去式了

You really let me down!
因为这样说最符合美国人或英国人的说话习惯,而disappoint一般用于书面语,做及物动词,后接人, sb. disappoint sb. 再按照情况选择时态和语态