In your code u missed end of statement that is ";" in the assignment sattement of nID. And also u missed specifying end of transaction.
Following is the correct code
nUser : string;
nID : string;
nUser := "some string";
if nUser = "some string" then
nID := "some string";//here mistake one
end TMain; //here mistake two
If u excute the above code string "some string" will be printed to the output.