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:
- We can use が when we first introduce a item.
- 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:
- The speaker just told the listener that her mother will make sushi.
- The listener already knows the speaker has a mother.