Does the が particle always mark the focus when marking the subject?

Question

I recently read lots of articles/posts from the が particle usages and there is something which isn't clear for me. So the basic usage of the が is to mark the subject of the sentence. But in addition が can be used to mark the focus of the sentence and in this case it is an exhaustive listing or neutral description.

First I think the が particle always marks the subject and the focus at the same time, because lots of article doesn't state the opposite. But seems that’s not true. The japanesewithanime.com says the following:

The ga が particle can mark a noun as having the focus in a simple sentence.

But what is the situation with the complex sentence? So after this I'm thinking lot and came the following conclusion:

When the が subject marker is used in a simple sentence or in a main clause then it marks the focus too. However が subject marker doesn't mark the focus too in the subordinate clause. Am I right?

So based on it the が is just a "plain" subject marker in the following example:

私 は　お母{かあ}さん が　作{つく}った　お菓子{かし} を　食{た}べました。

So what I want to know: is it possible the が is just a plain subject marker in the sentence?

Answer 1

You are correct. The subject of a main clause is typically marked with は, so when が is used instead, it carries a nuance such as exhaustive-listing or neutral-description.

お母さんは英語を勉強しています。
My mother is studying English.
^{(neutral; described as a known fact)}

お母さんが英語を勉強しています。
It's my mother who is studying English.
^{(nuanced: exhaustive-listing-ga)}

お母さんが英語を勉強しています。
(I found/report) My mom is studying English.
^{(nuanced: neutral-description-ga)}

On the other hand, the subject of a subordinary clause (including a relative clause) is marked with が by default, so it carries no extra nuance there. If は is used within a relative clause, that's probably a nuanced は known as contrastive-wa.

顔が美しい人
a person whose face is beautiful
^(neutral)

顔は美しい人
a person whose face, at least, is beautiful
^{(nuanced: indicates this person has some fault)}