Final answer:
Thaddeus Kosciuszko was the experienced Polish cavalryman who joined the Continental Army in 1777, contributing significantly to the American Revolutionary War as a military engineer and strategist.
Step-by-step explanation:
The experienced Polish cavalryman who joined the Continental Army in 1777 was Thaddeus Kosciuszko. The information provided discusses the invaluable assistance of Friedrich Wilhelm von Steuben, a Prussian soldier, who played a pivotal role in training the Continental Army into a disciplined and effective force.
Though von Steuben was crucial in this transformation, the specific question pertains to a Polish cavalryman, and that was Thaddeus Kosciuszko, who became an influential military engineer and strategist during the American Revolutionary War.
He is best known for his contributions to the fortifications at West Point and his leadership in several key battles.