tc-1:click file save(should open a window&should ask ur file name&path). tc-2:check for .extension like it shoule be .txt only. tc-3:check for short cut key like(ctrl+s) and ask for same option as above(should open an window and ask u the file name,path). tc-4:should accept all alpha-numerics.
Yes, we can write the test case for notepad if we know the basic functionality of notepad. We can group the test cases with main tab 1)File 2)Edit 3)Format 4)view 5)Help 6)Save new file 7)Memmory size file 8)Mapping with html files 9)changing file type html to txt and vice-versa. For each tab and functionality of each tab, we can write test case accordingly each.