> **He bought the house with a big backyard.** *This combination tells the listener which specific house he bought.*
> 
> **He bought a house with a big backyard.** *This combination tells the listener what kind of house he bought, but not the specific house he
> bought.*

I always thought that I can say "the house with a big backyard" whether the listener knows about the house or not.

But in the previous question regarding articles, I received an answer saying "**"the" won't work if the listener does not have any information about the house before hearing about it**", and said one should use "a" if the listener does not have any background information about it.

So I realized that "a" and "the" are entirely governend by whether the listener already knows it or not, not whether you are trying to specify the item or not.

I was living happily with this knowledge, until another question came into my mind.

Take a look at this conversation:

> John: What did you do yesterday?
> 
> Sam (a prosecutor): I was hearing *a reason a murderer killed the
> victim* being justified.

John did not know anything about the trial, and "reason the murderer killed the victim" is new information to him. John did not know about it till Sam told him. Thus, in this case, use of indefinite article should be natural.

But it doesn't sound natural. It sounds strange.

My gut tells me that "the" should be used in that case, even though the idea is introduced for the first time, and the listener knew nothing about it.

**I think the only time that "a" can be used in this sentence with "reason" is if there were more than one reason. But there was only one reason.**

But why can I not use "a" in that case? Or is it actually possible?

(the dialogue is made up)