Form1 f1 = new Form1();
            if (f1.checkBox1.Checked == true)
            File.AppendAllText("R.txt", f1.checkBox1.Text);

 public partial class Form2 : Form
    {
        Form1 f1;
        public Form2()
        {
            Form1 f1 = new Form1();
            InitializeComponent();
        }

        private void Form2_Load(object sender, EventArgs e)
        {
            if (f1.checkBox1.Checked == true)
            File.AppendAllText("R.txt", f1.checkBox1.Text);
        }
    }
}