keydown in c# doesn't work for some reason

I'm trying to do a calculator and all I have to do is make it work with the keyboard. This should work but it doesn't.

    private void Form1_KeyDown(object sender, KeyEventArgs e)
    {
        if (e.KeyCode == Keys.D1)
        {
            resultarea.Text = "fgdgd";
            //number(1);

        }
    ....
    }

Any idea what could have gone wrong?

Edit: it's still not working. I figured I might aswell post the whole code.

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
        KeyPreview = true;

    }
    double first = 0;
    double second = 0;
    string op = "";
    bool last = true;

    public void calculate()
    {
        double aux = first;
        first = second;

        if (op == "+")
            second = aux + second;
        else if (op == "-")
            second = aux - second;
        else if (op == "*")
            second = aux * second;
        else if (op == "/")
            if (second == 0)
                { first = aux; viewarea.Text = "Cannot divide by zero"; }
            else
                second = aux / second;
        //if (viewarea.Text == "Cannot divide by zero")
            resultarea.Text = second.ToString();

    }

    public void number(int digit)
    {
        if (last == true)
        {
            resultarea.Clear();
            last = false;
            first = second;
            second = 0;
        }
        resultarea.Text += digit.ToString();
        second = second * 10 + digit;

    }

    public void sign(string sign)
    {
        if (last==false && (first != 0 && second != 0))
            calculate();
        op = sign;
        debug.Text = op;
        last = true;
    }

    private void equal_Click(object sender, EventArgs e)
    {
        if (last == false && (first != 0 && second !=0 ))
            calculate();
        op = "";
        last = true;
    }

    private void one_Click(object sender, EventArgs e)
    {
        number(1);
    }

    private void two_Click(object sender, EventArgs e)
    {
        number(2);
    }

    private void plus_Click(object sender, EventArgs e)
    {
        sign("+");
    }

    private void minus_Click(object sender, EventArgs e)
    {
        sign("-");
    }



    private void three_Click(object sender, EventArgs e)
    {
        number(3);
    }

    private void creset_Click(object sender, EventArgs e)
    {
        first = 0;
        second = 0;
        op = "";
        last = true;
        viewarea.Text = "";
        resultarea.Text = "";
        debug.Text = "";
    }

    private void four_Click(object sender, EventArgs e)
    {
        number(4);
    }

    private void five_Click(object sender, EventArgs e)
    {
        number(5);
    }

    private void six_Click(object sender, EventArgs e)
    {
        number(6);
    }

    private void seven_Click(object sender, EventArgs e)
    {
        number(7);
    }

    private void eight_Click(object sender, EventArgs e)
    {
        number(8);
    }

    private void nine_Click(object sender, EventArgs e)
    {
        number(9);
    }

    private void zero_Click(object sender, EventArgs e)
    {
        number(0);
    }

    private void divide_Click(object sender, EventArgs e)
    {
        sign("/");
    }

    private void times_Click(object sender, EventArgs e)
    {
        sign("*");
    }

    private void sqr_Click(object sender, EventArgs e)
    {
        second = Math.Sqrt(second);
        resultarea.Text = second.ToString();
    }

    private void cos_Click(object sender, EventArgs e)
    {
        second = Math.Cos(second);
        resultarea.Text = second.ToString();
    }

    private void sin_Click(object sender, EventArgs e)
    {
        second = Math.Sin(second);
        resultarea.Text = second.ToString();
    }

    private void tan_Click(object sender, EventArgs e)
    {
        second = Math.Tan(second);
        resultarea.Text = second.ToString();
    }

    private void nfac_Click(object sender, EventArgs e)
    {
        double aux = second;
        if (second == 0)
            aux = 1;
        else
        for (double i = 1; i < second; i++)
            aux= aux*i;
        second = aux;
        resultarea.Text = second.ToString();
    }

    private void log_Click(object sender, EventArgs e)
    {
        second = Math.Log(second);
        resultarea.Text = second.ToString();
    }

    private void ln_Click(object sender, EventArgs e)
    {
        second = Math.Log10(second);
        resultarea.Text = second.ToString();
    }

    private void xexp_Click(object sender, EventArgs e)
    {
        second = Math.Pow(second ,2);
        resultarea.Text = second.ToString();
    }

    private void dot_Click(object sender, EventArgs e)
    {

    }

    private void Form1_KeyDown(object sender, KeyEventArgs e)
    {
        if (e.KeyCode == Keys.D1 || e.KeyCode == Keys.NumPad1)
        {
            viewarea.Text = "fgdgd";
            //number(1);


        }
        else if (e.KeyCode == Keys.D2 || e.KeyCode == Keys.NumPad2)
        {
            number(2);
        }
        else if (e.KeyCode == Keys.D3 || e.KeyCode == Keys.NumPad3)
        {
            number(3);
        }
        else if (e.KeyCode == Keys.D4 || e.KeyCode == Keys.NumPad4)
        {
            number(4);
        }
        else if (e.KeyCode == Keys.D5 || e.KeyCode == Keys.NumPad5)
        {
            number(5);
        }
        else if (e.KeyCode == Keys.D6 || e.KeyCode == Keys.NumPad6)
        {
            number(6);
        }
        else if (e.KeyCode == Keys.D7 || e.KeyCode == Keys.NumPad7)
        {
            number(7);
        }
        else if (e.KeyCode == Keys.D8 || e.KeyCode == Keys.NumPad8)
        {
            number(8);
        }
        else if (e.KeyCode == Keys.D9 || e.KeyCode == Keys.NumPad9)
        {
            number(9);
        }
        else if (e.KeyCode == Keys.Divide)
        {
            sign("/");
        }
        else if (e.KeyCode == Keys.Subtract)
        {
            sign("-");
        }
        else if (e.KeyCode == Keys.Add)
        {
            sign("+");
        }
        else if (e.KeyCode == Keys.Enter)
        {
            sign("=");
        }
    }
    private void Form1_Load(object sender, EventArgs e)
    {

    }


}

Assuming you have a WinForms project, set the KeyPreview property of your form to true, like that (e.g. in the constructor) :

public Form1()
{
    InitializeComponent();
    KeyPreview = true;
}

and it should work like you expected.

Edit: Due to your comment I've added the code to catch all the signs (add that to your Form1_KeyDown method)

if (((e.KeyCode == Keys.D7) && (e.Modifiers == Keys.Shift)) || (e.KeyCode == Keys.Divide))
{
    sign("/");
}
else if (((e.KeyCode == Keys.Oemplus) && (e.Modifiers == Keys.Shift)) || (e.KeyCode == Keys.Multiply))
{
    sign("*");
}
else if ((e.KeyCode == Keys.OemMinus) || (e.KeyCode == Keys.Subtract))
{
    sign("-");
} 
else if ((e.KeyCode == Keys.Oemplus) || (e.KeyCode == Keys.Add))
{
    sign("+");
} 
else if (e.KeyCode == Keys.Enter)
{
    sign("=");
}