Distinguish the topic from information being emphasized.
The rule
は introduces the topic we're talking about. が often identifies new information or answers "who? / what?". Both are not always interchangeable.
Examples
だれがきますか。
Who's coming?
たなかさんがきます。
Mr. Tanaka is coming.
Remember
- が is often used to identify what answers a question.
- Distinguish the topic from information being emphasized.
Common mistake
Use は systematically because French uses "le" or "la". The distinction depends on the information transmitted.