// add this private field 
private Form2 f2 = null;

// within a button click handler, say
private void button1_Click(object sender, EventArgs e)
{
    if (f2 == null)
    {
        f2 = new Form2();
        f2.FormClosed += Form2_FormClosed;
    }
    f2.ShowDialog();
}

// add this handler to main form manually, not from designer
private void Form2_FormClosed(object sender, FormClosedEventArgs e)
{
    // get text from textbox1 on Form2
    TextBox tb = (TextBox)f2.Controls["textBox1"];
    // display it in a label on main form, say
    label1.Text = tb.Text;
}