One of my teachers explained the following sentence that made me confused. I wanna know if he was correct or not. (I have a/one son)
This is what he said:
A-I have one son~ It means there is one or more daughters, as well. But there is only one boy and that boy doesn't have any brothers. My daughter(s) have only one brother.
B-I have A son~ I only have one son, and no other children. He is my only child. I have no daughters.
Was he correct about these explanations???
No. He was not correct. "I have one son" carries no implication by itself, implied or otherwise, about how many daughters there are in the family. It depends upon context.
Q1: "How many children do you have?"
A1: "I have one son."
This would be interpreted as meaning "I have one son - he is an only child."
A2: "I have a son."
This would be interpreted in the same way as A1, but is a strange way of replying.
A3: "I have one son and two daughters."
A4: "I have a son and two daughters."
A3 and A4 both mean the same thing, and are equally acceptable. Neither form is more common than the other in native speech.