What did hitler do that caused 3,000-4,000 civilian deaths each day in leningrad?

The German army, operating under Hitler's orders, enforced a military blockade against the city of Leningrad.  Starvation conditions in the city led to the 3,000 to 4,000 deaths per day mentioned in your question.
The siege of Leningrad lasted nearly 900 days and cost millions of lives.  It ranks among the most devastating of military encounters in history.

United States was an ‘associate power’ and it was not an official ally but fought against the central powers in World War I

Explanation:

The allies were the countries formed by triple alliance and the United States which battled against the dominant central powers. Triple entity called triple Entente was formed which means that there was an informal indulgence among Britain, Russia and France which was formed to oppose the central powers. Japan articulated its detachment and open mindedness but later joined the allies.

Associate powers were United States but later on it joined the allies and fought against the Axis powers. Associate members included Serbia and Romania too.