I scored very low. So I copied and pasted the answers for you. Call it a cheat sheet.
Happy: ㅎㅎ comes from 하하 (haha) and it represents laughter. ㅇㅋ comes from 오키 (oh-kee), which is a short form for 오케이 (Okay). ㅋㅋㅋ comes from 크크크, (kekeke)which represents the giggling sound, so it has the same meaning as ㅎㅎㅎ ^^ represents a smiling face, ㅍㅎㅎ is 푸하하 and has the same meaning as ㅎㅎ or ㅎㅎㅎ.
Sad: ㅉㅉ comes from 쯧쯧, which shows disappointment or sympathy (as in Tsk Tsk) ㅜ.ㅜ represents a crying face, with tears flowing down the cheeks from two closed eyes. ㅠ_ㅠ is a sad face with tears dropping. ㅡ.ㅔ represents a face of someone who is not excited and has long hair covering half of their face. -_- is also a sad look, ;;; represents sweat or embarrassment.
Other: ㅇㅇ comes from 응 (eung)which indicates yes ^^a, the letter "a" represents a hand and a finger scracthing one's head. ㄷㄷㄷ comes from the word 덜덜덜 (duhl, duhl, duhl), which describes a way in which someone shivers or trembles from shock or amazement. (equivalent to OMG).