What flaws of the Articles of Confederation did the Constitution fix?

Under the Articles of Confederation, the states were controlled loosely by a national legislature. This national legislature could not tax. There was no chief executive. The national government could only request taxation from the states--each state only wanted to pay its own debt. There was also no national currency.

The Constitution made federal law over state law. In Article I of the Constitution, Congress can regulate interstate commerce. It is also responsible for coinage and only Congress can regulate tariffs; in the early days of the national government, it was largely funded through land sales, excise taxes, and tariffs. Article II gave the national government a chief executive whose job it was to execute the laws. He had veto power over Congress, but Congress could override his veto or even vote to impeach him in extreme cases. To ensure that Congress would not go against the Constitution, the Founders even put in a Supreme Court to ensure that the laws were constitutional. All of this was done in order to have a functional national government which would act as a cohesive financial unit and pay down the national debts as well as safeguard the civil liberties of the people.