I would like to create a .txt file and write to it, and if the file already exists I just want to append some more lines:
string path = @\"E:\\AppServ\\Examp
You don't actually have to check if the file exists, as StreamWriter will do that for you. If you open it in append-mode, the file will be created if it does not exists, then you will always append and never over write. So your initial check is redundant.
TextWriter tw = new StreamWriter(path, true);
tw.WriteLine("The next line!");
tw.Close();