What's the difference between wa (は) and ga (が)?

Question

When is it correct to use は but not が, and when is it correct to use が but not は? Are there any times when you can use either without changing the meaning of the sentence? How does switching change the meaning of a sentence?

Answer 1

"が” tells us the subject of the verb; that is, who or what performs the action.

“は” is the topic marker of a sentence. This means that it tells us what we are talking about or what is being talked about in the sentence or clause.