Explanation
-(으)시- slots between a stem and its ending to honour the person doing the action, never the speaker: -시- after a vowel (가다 → 가세요, 가셨어요), -으시- after a consonant (읽다 → 읽으세요, 읽으셨어요). With -어요 it fuses to -세요/-셔요, with -었- to -셨-, with -ㅂ니다 to -십니다. Some verbs swap the whole word instead (먹다 → 드시다, 자다 → 주무시다, 있다 → 계시다). 할머니께서 오셨어요. — Grandmother has arrived.