Question: How can we write testcases from Requirements..?
Do the Requirements represent exact Functionality of AUT
Answer: Yes, Requirements should represent exact functionality of AUT.
First of all you have to analyse the requirement very throughly in terms of functionality. Then you have to think about suitable testcase design techniques (Black Box design techniques like Equivalance Partitioning, Boundry Value Analysis, Error Guessing and Cause Effect Graphing) for writing the testcase.
By these concepts you should write a testcase which should have the capability of finding the absence of defects.