Imperative basically means that you're telling someone to do something. For this purpose, use the ending ㅂ시오. Most of the time we use the honorific marker 시 before it, and for good reason. When you're using formal style, 90% of the time you're talking to your teacher, boss, military leader, or just someone you have to be nice to. So it makes them feel good that you put the 시 in there. Especially when you're telling them to do something. The 시 infix changes the meaning from "do ..." to "please do ...". Let's look at some examples:
- 안녕히 계십시오.
- Good-bye. (To someone staying.)
- (peacefully stay please)
- 안녕히 가십시오.
- Good-bye. (To someone leaving.)
- (peacefully go please)
- 케이크 드십시오.
- Please have some cake.
- (cake eat please)
- 연필 주십시오.
- Please give me a pencil.
- (pencil give please)