Originally Posted by RadioKid
>ga is used to introduce something new.
I cannot accept this definition. Do you have any example?
The Particle Wa I -
The topic is, thus, “old information.” In order for something to be registered information, though, you may need to use ga が first to establish it. Essentially, information needs to be new before it can be grammatically treated as old information.
Mukashi mukashi, aru tokoro ni, ojiisan to obāsan ga sunde imashita. Ojiisan wa yama e shibakari ni, obāsan wa kawa e sentaku ni ikimashita.
This sentence is the opening to one of the most important fairy-tales of Japan, Momotarō 桃太郎. At the beginning, the reader doesn't know anything about the story. This is why the particle ga が is used to mark the subject. Once the characters are established, they are then treated as the topic in the following sentence, thus marked by wa は.