Textbox input and output in C#

Hello,

I'm trying to enter some string in textbox1 and get modified output in textbox3, I wrote some code but it doesn't work. both textboxes are on their own and nothing happens when I enter the text.

namespace TextConverter
{
    public partial class TextConverter : Form
    {
        public TextConverter()
        {
            InitializeComponent();
        }

        private void textBox1_TextChanged(object sender, EventArgs e)
        {
            textBox3.Text = "";
            foreach(char s in textBox1.Text )
            {            
                if (s == 'a' || s == 'A')
                {
                    textBox3.Text += 'ა'; break;
                }
                // here goes some else-if statements.
                else textBox3.Text += s;
            }
        }
    }
}

Can you tell me what I'm doing wrong? do I need to change some form properties?

this is the first time I have something to do with windows forms and I have no idea how the things work.

Thanks in advance