有關形容詞(adjective)的應用...

其實Adjective是指用來形容 名詞 / 代名詞 (noun/pronoun)的字詞~
所以是有很多種類的~
e.g.
[性質] Adjective of Quality = kind, cruel, small, big, tall, green, red...
[數/量] Numeral Adjective (可數) + Quantitative Adjective (不可數) = any, some, many, much, few, little...
[指示] Demonstrative Adjective = this, that, these, those, such, same...

而用來形容名詞 / 代名詞的短句就稱為 Adjective Clause (/relative clause)~
i.e. The boy who was sitting there is my brother 中的 "who was sitting there"~

還有更多種類請看~
http://www.npc.edu.hk/staff/~ngtungying/english_grammar/
(Lesson 2,12,13,14,18,20,28,31,37,43~)

而你提到的 I feel sorry~
其實sorry正是形容I~確實點是I的"感覺"~
這和feel的特別詞性有關係~
這裏的feel其實是是感官動詞~ 後面通常接形容詞來形容感覺~
(參考：http://www.eng.fju.edu.tw/etc/quiz/common%20files/F.htm)
和Verb to be (is,am,are,was,were)類似~
某程度上你可當成是名詞/代名詞和形容詞之間的連接詞~
是用來更確切地指明形容詞是在形容名詞/代名詞的什麼特性~
例如 I feel cold / ashamed / bad...
I am hungry / worried...
留意是會有 I feel badly的~ (不過沒有 I feel sorrily / coldly...)
不過這次的 badly (adverb) 就真的是形容動詞 feel 了~
i.e. I feel badly 意思上等同 我的"感覺/觸覺"變差了...
請對比：I feel bad 意思上等同 "我"感覺很差...

Hope this helps~~
圖片參考：http://l.yimg.com/f/i/tw/ugc/rte/smiley_1.gif

sorry, sad, happy or upset is to describe our feeling . In the following sentence , it also has the function how we feel.

We feel sorry is not good English =

Proper English
We are sorry for......
We are upset that he cannot come or We are happy to do something .

We sometimes use adjectives to describe our feelings, like cold, sick ect.
In this situation, we will use adj. instead of advers.