This may be helpfull to you,
you can get values of check box in following way,
bool didYouCheckTheBox = myCheckbox.IsChecked;
Now remaining is that,
Just handle it working on button click.
So full code you required may be like,
buttonClickFunction()
{
bool didYouCheckTheBox = myCheckbox.IsChecked;
if(didYouCheckTheBox)
{
TextBox1.Text = "Checked";
TextBox2.Text = "";
}
else
{
TextBox1.Text = "";
TextBox2.Text = ""Un Checked";
}
}