Where did it go? 어다 갔니?
I lost it. 네가 그것을 일어버렸어(For losing an object not forgetting something or going crazy)
I found it. 찾았다.
This movie is really good! 이 영하는 친차 좋다!
To say hi is 안영.
To tell someone to say hi is 인사해줘 (this last one is a command so it can be rude, so if meeting them for the first time add주세요. If you really want to show respect then instead of commanding them by saying 인사해주시겠세요? Especially if they are older than you.) These are all informal, so use with care and only with kids, friends, or when talking to yourself.