In the sentence, "母がすしを作ります。", why do we use が instead of は?

Question

My book sentence: 母がすしを作ります。

I consulted another book, which states:

The case particle ga can mark the subject of the sentence. However, when the subject noun is also treated as the topic of the sentence, being marked by wa, ga cannot be present. That is a part of the reason why the particle wa often wrongly gives the impression of being the subject marker.

As I continued reading, from what I've understood, the context is insufficient, there is none actually. In fact, I think that we can use either particle for the following reasons:

1. We can use が when we first introduce a item.
2. We can use は when when the item was previously introduced or is already known.

How do we apply the above to the sentence? We may consider two cases:

1. The speaker just told the listener that her mother will make sushi.
2. The listener already knows the speaker has a mother.

Answer 1

Basically you can think 母 is always in "the universe of discourse", because when you say 母, it's determined; it almost certainly refers to your own mother. This means は is the ordinary particle choice. Usually, you should say 母はすしを作ります, replacing が with the topic-marker は.

However, が can be used in the following two cases:

• When exhaustive-が is intended.

  母がすしを作ります。
  It's my mother who makes sushi.
  _{(as a response to "Who makes sushi?")}

• When neutral-description-が is intended.

  母がすしを作ります。
  (I just noticed) my mother will make sushi!
  _{(when you are told to report everything happening in your house)}

I think your textbook will soon explain these special usages of が, but you can read several previous questions:

• What's the difference between wa (は) and ga (が)?
• Can someone explain me the use of は and が in this sentence?
• Why is this sentence ungrammatical? 「お寺が公園のとなりです。」
• Why does 「電話は切れた」 sound more adversarial than 「電話が切れた」?

Either way, if there is no previous context at all, 母がすしを作ります sounds unnatural and sudden. But textbooks sometimes need to show you this kind of "un-topicalized" sentence to explain how the Japanese sentence is structured.