What is "new information" for が?

Question

I've been trying to understand the age old question of が vs は recently. The resources I've used so far have given helpful answers, but I'm still not getting 1 key point. I'll summarize my knowledge so far:

Tae Kim (see this and this) points out that が is mainly an identifier particle, i.e. it identifies new information. Makino and Tsutsui note this as well in their book (see this, pp118-20). This excellent post on this site talks about it as well and uses the concept of “universe of discourse” to indicate what can and cannot use は and が. If it’s new, i.e. not in the universe of discourse, then it uses が. If not, it uses は. None of these sources, while helpful, can fully answer my questions however.

I'll give some examples that are confusing me. My potential solution is that context determines when the bellow use は or が, but I don’t understand how it would.

(a) The classic example of the beginning of a fairytale that many, including Makino and Tsutsui, use [pp118-9]:

昔々一人のおじいさんが住んでいました。おじいさんはとても貧乏でした

This is perfectly clear. We don't know the old man, so he's introduced via が and then は can and must be used. Same as in English the and a.

There once was an old man. The man was very poor.

What confuses me, however, is generic nouns and some pronouns (see next examples) where I can't use the "a and the" test.

(b) Makino and Tsutsui give the following example:

雨が降っている

I don’t understand here how 雨 constitutes “new” information. Makino and Tsutsui say that は can be used with “generic nouns” that are not new information by virtue of their being general knowledge (their examples are “車” and “人”). Wouldn’t this include something like "rain"?

(c) In this Stackexchange post, Flaw (question author) considers and example dialogue: "

Person A: どちらが東ですか。Person B: こちら（ は / が ）東です。

This is an exercise, and according to the person answering the question, it's が as こちら constitutes "new information". The answer there doesn't sufficiently explain, for me to understand, why this is new information. As Flaw points out, isn't this part of the environment the people are speaking in, and thus context should give have introduced こちら already? Also, I don't understand in general how a generic non question-word pronouns like こちら can be sometimes "new" and sometimes "old" information (as both は and が are used with it at different points). Questions words make sense, as they’re always unknown information, but other pronouns (including things like 私) don't make sense to me.

Again, my potential solution is that context determines when the bellow use は or が, but I don’t understand how that would happen.

ありがとうございます! よろしくお願いいたします!

Answer 1

When you find a theory doesn't get along with what's observed in reality, there's no point in sticking to that theory.

Cases where you should use が or other case particles without topicalized in a sentence can fall into three categories.

1. Sentence of neutral description
2. Exhaustive listing
3. A part of coherent sentences

Exhaustive listing

"It's apple that's red" can be translated into 赤いのは りんごだ, and you can rephrase it as りんごが赤い. Likewise, うなぎは 私だ (it's me who ordered eel) can be rephrased as 私が うなぎだ. This kind of usage is called exhaustive listing.

Coherent sentences

Instead of saying 生徒が頭が痛いのでは 練習にならない (When pupils have headache, it can't be enough training), you can express it as 生徒が頭が痛い、これでは練習にならない. Now, the part 生徒が頭が痛い in the second sentence looks a sentence without topic. But it's not really an independent sentence per se.

In your examples, you can explain おじいさんが住んでいた and 雨が降っている with Sentence of neutral description, and こちらが東 with Exhaustive listing (you can rephrase it as 東はこちら).