How did Hitler conquer so much of Europe in such a short time?

Since you use
the word conquer, I assume that you are asking about the actual military conquests of various
European countries, not the peaceful (though coerced) absorption of places like Austria and
Czechoslovakia.  The German Army conquered Poland, Belgium, the Netherlands, France, Denmark,
and Norway all within the first few months of the war.  It then conquered much of the Soviet
Union in the early days of Operation Barbarossa in the summer of 1941.

There
were a number of reasons why it was able to do so.  Let us look at three of them.

First, the French and the British had not been prepared to fight a war.  Therefore,
they were not really able to do anything to prevent Germany from taking countries like Poland
and Norway.  They simply did not have the necessary military power ready to go when these
invasions occurred.

Second, some of these countries were simply too small to
put up any kind of a serious resistance to German invasions.  Even in the best of times, a
country like Belgium or Denmark would never be able to resist a major power like Germany for
very long at all.

Third, the German military had developed its new tactic of
blitzkrieg.  This was a new development in offensive tactics.  It combined mechanization of as
many units as possible (for fast movement) with close coordination between tanks, foot soldiers,
and air support.  The armies of France and England were not ready for such tactics, which is why
even France fell as fast as it did.

These are the main reasons for Germany's
ability to conquer so much so quickly.