In Korea, we think polite to elders and adults are very important. So, there are difference between words for using elder.seniors.adults and friends and junior. someone who is younger than me. We use 'Saranghae' and 'Saranghanda' to friends and younger.However, we usually use Saranghanda poetry or novels which is more appropriate for formal words. So, if you want to express fellowship or love to others, use Saranghae. Saranghandago is quiet funny to say haha. We also use this for friends or young. If you use this to elder or who are older than you, it can be rude. Also, I hear this word for drama. Usually cold guy use this for woman. Use Saranghaeyo to elders and adults. This word is expressing polite! If you want to know more please ask me. Hard work in Korean!