The constitution establishes the valid purposes for the legislature to make laws. It concerns itself with ensuring that the government is not tyrannical by then allowing the executive branch to veto a tyrannical law. Through the balance of power established by the constitution, tyrannical laws are winnowed out.
The second question, "What are the functions and sources of American Law?" focuses attention even closer on the manner by which the United States government derives its authority to govern its population. The legislative branch makes laws using a process defined by the constitution. However, what establishes the kinds of laws that the legislature can make? Once again, the constitution, which is the basis the United States government, takes responsibility. It enumerates the subjects on which the legislative branch can make laws. Over the years, a lot of new meanings have been interpreted from these enumerations. However, this is necessary to some extent due to the changing levels of technology and the acceleration of progress. For example, it is doubtful that the framers could foresee the need for laws regarding the governance of email. It must be remembered that at the time of its framing, electricity in its current harnessed form was one hundred years away, at the least. Thus interpretation of the constitution has had to be loose at times.
The sources of American law are not plural. There is one source and it is the legislative branch. This branch is responsible for putting the laws on the books for the executive and its bureaucracy to enforce. Laws come into being by following a constitutionally defined procedure. The first step that is taken to pass a bill into law is committee action. A subcommittee writes the bill, which is then taken to a committee. The committee then edits the bill. It should be stated that both houses have separate committees, often resulting in differences in the content of the bill.