An nyoung ha seh yo!
That's hello. In Korean. Written in Hangul, that's 안녕하세요.
This is a crucial phrase if you want to get by in Seoul. Everywhere I walk, I hear "an nyoung ha seh yo!" People answering cell phones, passing friends on the street, bowing to elders and superiors in the hallway at work. When one walks into any store or restaurant in Korea, he or she is greeted with a chorus of "an nyoung ha seh yo!" from any employees in the vicinity of the door.
Also, a phrase I've recently learned, and put to very good use:
This means "thank you." I got into the habit of using this today, much to the delight of the Koreans I interacted with. They were so happy when I used it, and really made me feel more at ease about trying different Korean phrases out. I purchased a bottle of orange juice at the Mini-Stop convenience store near my officetel, and when I couldn't quite remember the phrase, the store clerk had no problem with helping me out a bit. She asked where I was from, and welcomed me to Korea. Many people here are so inviting, it really makes me feel very welcome.