> The particle "ga" and "wa" both introduce a topic
Forgive me for saying this, since you seem to be a native speaker, but don't you mean that they both introduce the subject, not topic (using 'topic' as a linguistic term)?
"Wa" would be the topicalising subject marker, denoting known information:
Tanaka wa nihon ni itta.
Tanaka went to Japan. -> As for Tanaka, he went to Japan.
Tanaka = known information (i.e. Tanaka is familiar to the listener)
"Ga", while also a subject marker could denote/introduce new information:
Tanaka ga nihon ni itta.
Tanaka went to Japan. -> e.g. It was Tanaka who went to Japan.
Tanaka = new information (e.g. the listener is did not not Tanaka was the one going to Japan.)
(Note: I realise there are other constructions for my interpretation of the ga-sentence)
Forgive me for saying this, since you seem to be a native speaker, but don't you mean that they both introduce the subject, not topic (using 'topic' as a linguistic term)?
"Wa" would be the topicalising subject marker, denoting known information:
Tanaka wa nihon ni itta.
Tanaka went to Japan. -> As for Tanaka, he went to Japan. Tanaka = known information (i.e. Tanaka is familiar to the listener)
"Ga", while also a subject marker could denote/introduce new information:
Tanaka ga nihon ni itta.
Tanaka went to Japan. -> e.g. It was Tanaka who went to Japan.
Tanaka = new information (e.g. the listener is did not not Tanaka was the one going to Japan.)
(Note: I realise there are other constructions for my interpretation of the ga-sentence)