There are several different words for Hello depending on context in the Korean language.
English: Hello (most common)
Korean: 안녕하세요 (annyeonghaseyo)
Sounds Like: an-nyong-ha-say-o
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English: Hello (very formal)
Korean: 안녕하십니가 (annyeonghashipnikka)
Sounds Like: an-nyong-ha-shim-ni-kah
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English: Hi (Informal/Among Friends. It actually means peace or be well)
Korean: 안녕 (annyeong)
Sounds Like: an-nyong
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English: Hello (answering the phone)
Korean: 여보세요 (yeoboseyo)
Sounds Like: yo-bo-say-o