Based on the level of respect or formality, you can say "He is not in the office" in Korean in 3 ways:

• Informal: geu-neun samusil-e eopseo (그는 사무실에 없어)
• Formal: geu-neun samusil-e eopseoyo (그는 사무실에 없어요), geu-neun samusil-e an gyeseoyo (그는 사무실에 안 계세요), geu-neun samusil-e eopseupnida (그는 사무실에 없습니다)
• Very Formal: geu-bun-eun samusil-e an gyesipnida (그분은 사무실에 안 계십니다)

- he (subjective): geu-neun (그는), geu-bun-eun (그분은)
- be not: eopda (없다)
- be: itda (있다)
- office: samusil (사무실)
- in the office: samusil-e (사무실에)

Please refer to How to pronounce Korean Alphabet for the English pronunciation in this answer.