Do 같이 and 함께 mean the same thing? If they are different then how do I know which one to use?
For the meaning "together", they are interchangable, but I'd say 같이 is more colloquial.
같이 has other meanings though. These are not interchangable with 함께.
- like(adverb) : 선생님이 하는 것과 같이 하세요. (Do it like the teacher does it)
- like(particle) : 얼음장같이 차가운 방바닥 (room floor as cold as ice)
- emphasizing a time : 새벽같이 떠나다 (leave in the early morning)