Period 1 begins with 1491. if the american indian population in what is now the united states was nearly 10 million before 1492, why is the united states population in modern times only 2 to 3% american indian?

That is because the vast majority of Native American people died for various reasons. Mostly it was about diseases. Europeans brought diseases that they had immunity too like pox and they didn't know that the Natives didn't have that immunity, so they started dying because of it. Many also died in wars against the Europeans, but the highest part was due to viruses.