세요 is a short form of "시어요"
안녕하세요 is composed of
안녕하(다)+시+어요.
시 is an honorific particle.
어요. is an ending particle used for imperative, interrogative or declarative sentence.
시어요 or 세요 is an honorific ending used for imperative, interrogative or declarative sentence.
So 안녕하세요 is an honorific expression of 안녕해요
Past tense of 안녕하다 is 안녕했다.
Past tense of 안녕해요 is 안녕했어요.
Past tense of 안녕하세요 is 안녕하셨어요.