This one is suuuuper useful, I would use it every day especially when ordering coffee or lunch! The basics for me were:
안녕하세요 - hello
감사하다 - thank you
I think it is also important to learn to recognise sentences that might be said to you:
잠시만요 - wait a moment
어서오세요 - welcome (when entering a shop, restaurant)
포장 - take out (food, coffee)
I do not speak Korean so this is only what I can remember 
but if someone could translate sentences like the following that would be nice! Like not to get too confused at the shop for example:
- Do you need a bag?
- Do you have the membership?
- Do you need a receipt
- For here or take out?