private void Form2_Load(object sender, EventArgs e)
    {
        CreditData helper = new CreditData();
        double cash = helper.sumCredit;
        double percent = helper._percent;
        if(helper.ann == 0)
        {
            double commonPay = helper.comm;
        }
        else
        {
            double coeffAnn = helper.ann;
            percent /= 12;// % 1 mnth
            percent /= 100;

            DataTable table = new DataTable();

            table.Columns.Add("Місяць", typeof(int));
            table.Columns.Add("Сума щомісячного платежу", typeof(double));
            table.Columns.Add("%", typeof(double));
            table.Columns.Add("Погашення основної суми кредиту", typeof(double));
            table.Columns.Add("Залишок позики", typeof(double));

            for(int k = 0; k < helper.termin; k++)
            {
                table.Rows.Add(k + 1, coeffAnn, percent * cash, coeffAnn - (percent * cash), cash - (coeffAnn - (percent * cash)));
            }
            dataGridView1.DataSource = table;
        }
    }

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }

    private void button1_Click(object sender, EventArgs e)
    {
        double start_sum;
        double interest_rate;
        double donation;
        int term;
        bool checking = true;
        CreditData helper = new CreditData();
        //test: Is you dork or no?
        try
        {
            term = Int32.Parse(textBox5.Text);
            start_sum = double.Parse(textBox4.Text);
            interest_rate = double.Parse(textBox1.Text);
            donation = double.Parse(textBox2.Text);
        }
        catch
        {
            MessageBox.Show("Error!nBe more attentive, please!nYou made a mistake!");
            checking = false;
        }
        bool checking_comboBox;
        bool checking_zero = true;
        if(checking != false)
        {
            if ((start_sum = double.Parse(textBox4.Text)) == 0
            || (interest_rate = double.Parse(textBox1.Text)) == 0
            || (donation = double.Parse(textBox2.Text)) == 0
            || (term = Int32.Parse(textBox5.Text)) == 0)
            {
                MessageBox.Show("Error!nYou cannot enter 0!");
                checking_zero = false;
            }
        }

        if (comboBox1.SelectedIndex > -1) checking_comboBox = true;
        else checking_comboBox = false;

        if (checking == false || checking_zero == false || checking_comboBox == false)
        {
            textBox1.Clear();
            textBox2.Clear();
            textBox3.Clear();
            textBox4.Clear();
            textBox5.Clear();
        }
        //end test
        else if (checking_zero == true && checking == true && checking_comboBox == true)
        {
            start_sum = double.Parse(textBox4.Text);
            interest_rate = double.Parse(textBox1.Text);
            donation = double.Parse(textBox2.Text);
            term = Int32.Parse(textBox5.Text);
            textBox3.Text = (start_sum = start_sum - donation).ToString();
            helper.sumCredit = start_sum;
            helper._percent = interest_rate;
            helper.termin = term;
            helper.ann = 0;
            helper.comm = 0;
            Form2 _show = new Form2();
            if (comboBox1.SelectedIndex == 0)
            {
                helper.ann = Annuity(start_sum, interest_rate, term);
                _show.Show();
            }
            else if (comboBox1.SelectedIndex == 1)
            {
                helper.comm = CommonMethod(start_sum, term);
                _show.Show();
            }

        }
    }
    double Annuity(double _startSum, double _interestRate, int _term)
    {
        double coefficientAnnuity = (_interestRate / 100) / 12 * Math.Pow((1 + (_interestRate / 100) / 12), _term) / (Math.Pow((1 + (_interestRate / 100) / 12), _term) - 1);
        coefficientAnnuity = _startSum * coefficientAnnuity;//тіло кредиту з відсотками
        return coefficientAnnuity;
    }
    double CommonMethod(double _startSum, int _term)
    {
        double monthlyPay = _startSum / _term;
        return monthlyPay;
    }
}

class CreditData
{
    public double sumCredit { get; set; }
    public double _percent { get; set; }
    public double ann { get; set; }
    public double comm { get; set; }
    public int termin { get; set; }
}