The difference is in speech levels. There are seven speech levels in Korean, and each level has its own unique set of verb endings which are used to indicate the level of formality of a situation. 알았다 and 다했다 are Haera-che(해라체) It is used to close friends or relatives of similar age, and by adults to children in impersonal writing (books, newspapers, and magazines) and indirect quotations ("She said that...") in grammar books, to give examples in some exclamations 알았어 and 다했어 are Hae-che(해체) It is used between close friends and relatives when talking to children -ㄴ다 is a present ending partile of Haera-che. So, 걸린다 and 간다 are Haera-che present of 걸리다 and 가다 Refer to http://en.wikipedia.org/wiki/Korean_speech_levels