procedure TForm1.Button1Click(Sender: TObject);

var   F:TextFile;

      fName:

string;

begin

    fName

:=

'error.txt';

    AssignFile(F,fName);

    

if FileExists(fName)  

then Append(F)  

else Rewrite(F);

    Writeln(F,

'this is string');

    CloseFile(F);

end;