This is a huge question that covers a very wide range of Korean grammar aha. To start off with, there are different levels of politeness in Korean, and this is indicated by the way a sentence ends (attached to the end of the final verb or adjective) - if it ends with 'yo' (요), this is a polite but informal sentence that someone might use to someone older than them or someone they don't know well. If it ends with '(b/sub)nida' (ㅂ/습니다)', this is a very polite and formal sentence that you would use in a formal situation. If the ending is different from the two I mentioned above, it is likely that it is an informal sentence that is to only be used with friends or those younger than you. On top of that, there are various different endings (which can be followed by 요 or ㅂ/습니다 to make them more polite) which have different grammatical meanings. If it ends with something that sounds like 'seo' as you mentioned above, it could be a past tense sentence or word (such as 갔어, 'gasseo' (went), which is the past tense of 가, 'ga' (go)). I really can't explain this all because your question is so broad, but basically it is all down to grammar and honorifics/politeness. If you want to learn more about Korean grammar, a great place to start is TalkToMeInKorean: http://talktomeinkorean.com