が

1. Origins, structure, and basic meaning

• Structure: Add が after the noun that is the subject of an action or state.
• Basic usage: In 猫が窓のそばで寝ています。, the cat is the entity sleeping.

2. Common pitfalls and similar expressions

• Answering who acted
  • Answering だれが来ましたか with 田中さんが来ました identifies who came. This is a context in which が identifies the subject.
• Compared with topical は
  • 田中さんは来ました makes Tanaka the topic and says that they came, possibly in contrast with others. Do not substitute が and は using only a simple new-versus-old information rule.