You say "Hello" as well, using 합쇼체 (hapsyochae), which is a high level of formal speech that is used for speaking to strangers and elders. It's used to show humble respect. Now it's 12:00 PM and your team goes out to a restaurant across the street for lunch. When you tell the waiter your order, you use an informal but polite form of speech: 해요체 (haeyochae). This is the most commonly used form in informal settings where respect is expected. Unlike many of the other forms, this can be used regardless of the rank of the other person (whether they're older, the same age or younger). After work, you meet up with your friends at a cafe. In this situation, you use 반말 (banmal). It's the most casual way of speaking and can be used between close friends, close family members and children. It is also used by adults to speak with children.
As you can see, the average Korean person may use three or more forms of Korean in a given day, and it's very important that they use the right form in each situation. Using too much respect may make you seem old-fashioned or make the other person feel uncomfortable, but it won't be taken offensively. Not using enough respect is a much worse mistake that can get you fired from a job or even ruin your relationship with others.
