Grammar · Particles
は or が, the short version
[Known] は … / [New] が …
Use は for something already in play, and が for something new or being singled out. 'Who came?' takes が in the answer, because the person is the new information. 'What about Tanaka?' takes は, because Tanaka is already the topic.
The mistake to avoid
Do not try to master this in week one. Copy the patterns you meet, and the feel arrives after a few months of input.
Examples
誰が来ましたか。
だれが きましたか。
Who came?
田中さんは来ました。
たなかさんは きました。
Tanaka came. (as for Tanaka)