-하고, -와, -과, -랑, and -이랑 are commitative markers in constructions like these. In English, the preposition with is used.
Mods. ..please allow sentences with or without 'a' or 'the'.... it's impossible to guess which (if any) you want! !!
There is kind of a rule for this though. Generally speaking: if the subject is marked with - 는 you have to use 'a' in your translation. 아버지는 = a father. If - 가 is used you have to translate with 'the'. 아버지가 = the father.
The father and the daughter are singing - what is wrong?
I explained it above. It only means and between nouns. After a subject is declared, the attachment of a 하고 marker indicates that the subject is doing something with the marked item.
Your sentence is different: 아버지하고 딸이 노래하고 있어요.
This sentence is literally:
Father(가) daughter with singing.
Translates to "The father is singing with the daughter."
"The dad sings with the daughter" should count
Why is it is singing and not sings?
The operative verb here is the present progressive form: 노래하고 있다
I misread the sentence as "The father follows along and sings" ( 아버지가 "따라고" 노래하고 있어요)