Final answer:
Thomas Jefferson contributed to the nation's founding by authoring the Declaration of Independence and orchestrating the Louisiana Purchase, which expanded U.S. territory.
Step-by-step explanation:
Two significant contributions Thomas Jefferson made to the building of our nation include drafting the Declaration of Independence and overseeing the Louisiana Purchase.
As the principal author of the Declaration of Independence, Jefferson articulated the colonies' reasons for seeking independence from Great Britain.
His role in the Louisiana Purchase, on the other hand, substantially expanded the territory of the United States, providing vast lands for future growth and exploration, such as the Lewis and Clark Expedition that he commissioned, which advanced scientific and geographic understanding.