は、が、を、に and information structure of Japanese Question

Question

I read 日本語の助詞(情報構造） and from what I understand it says:

What comes before は is old information and after it it's new information. (This makes sense to me).

What comes before が is new information (this makes sense to me) and after it it's old information (This does not make sense to me though).

So I tought of a conversation in my head.

My parents call me at the cellphone while I am in a bar with some friends.

Parents:お祖母さんは死んだ。

My friends ask me what's wrong and I answer: お祖母さんが死んだ。

I am not sure if this is correct to be honest, but this is how I think it would go.
My reasoning is that me and my parents already have a mental referent for お祖母さん and we assume she existed, therefore old information は. My friends do not know that I have a お婆さん so she is new information therefore が, but the fact that she died is new information too.

So from what I understand since は is the only old information what comes is all new information right?

Also by reading:

話題は「話者が区別し、それについての情報をコメントとして与える対象」である

It makes sense that you separate the old info from the rest and you add new info.

Can someone explain to me if I am wrong? From what I understand the only old information is marked by は (and も), while the new information is marked by all the other particles (がをにで and も included).

Answer 1

Whether some information is new or not is certainly one of the most important criteria, but that's not the only thing that determines which to use.

In this case, both your parents and you must say お婆さんが死んだ even though you know her well. See:

• Can someone explain me the use of は and が in this sentence?

  When you describe or report information which is newly discovered by perception that doesn't include assumption or judgement, Japanese grammar requires you to express it as a sentence whose elements are not topicalized, in short, without adding particles like は.

• Why does 「電話は切れた」 sound more adversarial than 「電話が切れた」?
• What's the difference between wa (は) and ga (が)?

  Any が can be an exhaustive-listing が, but neutral description only works with action verbs, existential verbs, and adjectives/nominal adjectives that represent state change. "Sentences of neutral description present an objectively observable action, existence, or temporary state as a new event."

After saying this, you may add something like 「うちのお婆さんは3年前から病気だった」「お婆さんは昨日まで元気だったのに…」 because these are not new events that just happened.

Answer 2

Generally, the usage “は” and “が” depends on where you focus on.

---

If you focus on your grandma, it’s natural to use “おばあさんは・・・.”

You don’t know how grandma is, whether she is alive or dead.

”おばあさんはどうしていますか？ How are grandma?”
“おばあさんは死にました。 Grandma has gone.”

In this context, “おばあさんが死にました” is unnatural.

---

If you focus on who, it’s natural to use “,XXXが・・・”
You know someone has gone, but don’t know who has gone.

”誰が死んだのですか？ Who has gone?”
“お婆さんが死にました。Grandma has gone.”

In this context, “おばあさんは死にました” is unnatural.

---

I'm not sure but after you get answer and you are aware of it. It must be old information. For me, the explanation of usage "は" and "が" by "new information (before you are aware of it)" and "old information (aftre you are aware of it)" is something wrong.