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

        private void button1_Click(object sender, EventArgs e)
        {
            ff.ShowDialog();
            if (ff.DialogResult == DialogResult.OK) MessageBox.Show("Test");
        }
    }

    public partial class Form2 : Form
    {
        public Form2()
        {
            InitializeComponent();
            textBox1.Text = "Original text";
        }

        private void button1_Click(object sender, EventArgs e)
        {
            this.DialogResult = DialogResult.OK;
            this.Close(); // или this.Hide();?
        }
    }