I am new in quartus. one of my home work was to implement a full adder with two half adder in quartus. now I created half adder but I don't know how to use it in other schematic file to implement full adder. thanks.
You have to export your half adder to a .bdf file (File > Create/Update > Create Symbol Files for Current File) and then use it as a symbol (i.e. just like other gates) in another schematic files.