안녕하다 - 안녕하세요 with what grammar has this been conjugated?
I thought it would use past tense + yo but then it should be 하했어요 at then end and not 하세요. Anyone knows what kind of grammar rule uses 세요 at the end? Thanks!

Question

안녕하다 -> 안녕하세요 with what grammar has this been conjugated?
I thought it would use past tense + yo but then it should be 하했어요 at then end and not 하세요.  Anyone knows what kind of grammar rule  uses 세요 at the end?  Thanks!

Answer

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.

Answer

-세요 is an imperative ending, used to tell somebody to do something. 하세요 means "Please do...".