께서 & 께

Two particles have an honorific form:

• 께서 replaces 이/가 when a respected person is the subject: 할아버지께서 신문을 읽으십니다 (Grandfather is reading the newspaper).
• 께 replaces 에게/한테 when a respected person receives something: 선생님께 선물을 드렸어요 (I gave the teacher a present).

To choose, look at the person's role and at the verb. If the person does the action, the verb carries -(으)시- and the particle is 께서. If the person receives something, the verb is usually 드리다 or -아/어 드리다 without -시-, because you are the one acting, and the particle is 께.

께서 is formal. In everyday speech many people simply say 할아버지가 읽으세요, and as a topic it becomes 께서는. Never use 께서 about yourself.