으시

honorific — raises the person acting

Slotted inside the verb to show respect for whoever is doing the action: 들으셨을 때 'when you listened', 있으신가요 'do you have', 수고 많으셨습니다 'you worked hard'. It appears as 으시 after a consonant and shrinks to 시 after a vowel (보시다, 가세요), and it fuses so tightly that the syllable often looks like an ordinary verb form. Because it honors the subject, you use it about the person you're addressing or a respected third party — never about yourself.

Register: polite/deferential