particle が - Neutral vs. Exhaustive が

The particle が has a property that implies that, of all items from some group, the sentences applies exclusively to the noun before it. For example, in a room of people from different countries, saying 「私がアメリカ人です」 would imply that you are the only American in the room.

However, it is also possible to use a more neutral が without this meaning. For example, 「雨が降る」 is neutral and isn't comparing the rain to anything.

So how do you know when the particle が carries this exclusive-definition and when it doesn't?

Answer

Warning... this answer presents things differently from the more complex and accurate model, which can be found here: http://www.lel.ed.ac.uk/~heycock/papers/topic-draft.pdf

I can't think of how to summarize the model in that paper to fit in an answer, so instead I'm going to present a different model (mostly from my intuition, so do take it with a grain of salt) which works for a more limited number of sentences, but I think lines up result-wise with the model in that paper.

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Taking a basic XがY clause...

The determiners of the semantics of が are:

1. whether Y is temporary or permanent,*



2. whether X is new or old information,


3. whether the clause is embedded or not

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When you have a temporary predicate, the neutral reading of が becomes possible.

雨が降っている。
"Rain is falling."

天気が寒い。

"The weather is cold."

牛が草を食べている。
"A cow is eating grass."

_{(Note: all of these can be made exhaustive by placing stress on the が-marked argument.)}

However, if the が-marked argument is already in discourse, then you would mark it by は.

「雨はどう？」「雨はまだ降っている。」 (You could, and normally would just drop 雨は entirely, but it's okay to have it, and necessary if you say some other stuff before answering the question. Using 雨が would sound weird, as if you aren't responding to the question but just making a statement.)

「今日の天気は寒いねー。」 (Using が would be exhaustive here)

「あの牛は草を食べている。」 (Using が would be exhaustive here)

There are some things which are in discourse by construction, such as 私, or things marked by その, あの, 今日の etc. -- so if they are accompanied by が the only way to read them is as exhaustive reading despite the possibility introduced by a temporary predicate.

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When you have a permanent predicate, it is necessarily the exhaustive reading of 「が」:

太郎が背が高い。

アメリカに生まれた人がアメリカ人だ。

牛が草を食べる。

So you instead mark these things with は to get a neutral reading.

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What I've said so far holds true at the top level of the sentence. However, in (most) subordinate clauses, the only reading available for は is the contrasitive reading, so instead things fall back to が (regardless of temporary/permanent or old/new distinctions):

太郎は私が好きな本を捨てた。

_{[*] These are called "stage-level predicate" and "individual-level predicate" in the literature.}