곰 세마리가 한 집에 있어 아빠곰 엄마곰 애기곰 아빠곰은 뚱뚱해 엄마곰은 날씬해 애기곰은 너무 귀여워 으쓱으쓱 잘 한다

you're right! its just another way to say "my name is__" It literally means: "i call myself___" but since it is 저는 and ends in ㅂ니다 , then it is very formal :)

"Indirect statements An indirect statement is one in which the speaker is reporting something that was said by another individual. (This form of speech, called indirect speech, is also referred to as reported speech.) The basic ending for indirect statements, -다고 하다, changes depending on the tense of the statement that is being reported. It also changes depending on whether a descriptive or an action verb was used in the statement that is being reported. a. Present D.V. + 다고 하다 A.V. + ㄴ/는다고 하다 N. + (이)라고 하다 b. Past V. + 었/았다고 하다 N. + (이)었다고 하다 c. Future V. + ㄹ/을 거라고 하다 N. + 일 거라고 하다 " information is from this site: http://www.language.berkeley.edu/korean/10/lesson05/index.htm

here's an explaination of ~더니: http://ezcorean.com/bb_index.php?level=1&subaction=vthread&topic=70203&forum=24&from_lang=korean&switch=p&old_cfile=mod_pager&ocfbv=browse_all&entrant=&wordid=203&pos=1&posn=1 ~다가 is like: while doing something, something interrupted ~다고 comes from 다고 하다 , it means "he/she said"