Explain what is meant by the topic, the main idea, and the purpose of the writing?

A purpose is when somebody has a specific reason for writing, a goal. To inform, entertain, explain the topic. for the specific piece of writing.

what is a theme?

ans: the implicit idea of a written work of literature is called a theme.

so, the only theme of "birdfoot's grampa" by joseph bruchuc is that as a human being one should always remember the value of every life on earth.

the grampa is stopping the car again and again to save those little toads from washing away in the heavy rain is an act of kindness and humanity.