How to use "if" function with DataGrid View [Repost for solution] [duplicate]

I've been searching with google/stackoverflow for a while, but I couldn't find proper answer for my problem.

I have to create simple program in VS Windows Forms (.net Framework). I have created few DataGridViews with showing exact data I need. I added buttons to "Add" to create new row, or "Remove" selected row. Now I need to change data in selected row with single button click. When I click "Add" button, new dialog window pop-up, where you choose from subclasses Car or Motorbike, then when you choose what to add, new dialog will pop-up, and there you fill proper information (like Manufacturer, or year of creation etc.)

When I want to "Edit" selected row, I want the app to directly go to dialog where you can change the data to new ones. I thought simple "if" could help me easily, but i cannot find the code to create it. My thought was something like this:

            F1Automobil f1 = new F1Automobil();
            F1Motorka f2 = new F1Motorka();
            int indCar = dgvVozovyPark.CurrentCell.RowIndex;
            if (/*index of selected row where subclass equals to "automobil" = true)
            {
                {
                    f1.Action = F1Automobil.ActionType.Edit;
                    f1.CarInstance = (Automobil)dgvVozovyPark.CurrentRow.DataBoundItem;
                    f1.ShowDialog();
                }
            else
                {
                    f2.Action = F1Motorka.ActionType.Edit;
                    f2.BikeInstance = (Motorka)dgvVozovyPark.CurrentRow.DataBoundItem;
                    f2.ShowDialog();
                }
            }

But VS shows me several mistakes. Could somebody please help, because i don't know what to do. I tried a lot of combinations like Database.Equals(something) = true, but none worked.

Also, I need help with other thing, so I will write it here, because I don't want to create new thread. I want to implement one if function for NumericUpDown. I will share code below:

        public void InitializeDate()
        {
            if (numMonth.Value.Equals(2) == true)
            {
                numDay.Maximum = 28;
            }
            else if (numMonth.Value.Equals(4 | 6 | 9 | 11) == true)
            {
                numDay.Maximum = 30;
            }
            else
            {
                numDay.Maximum = 31;
            }
        }

This code show no mistake or error, but it's not working properly. Could anyone help with this aswell ?

I would like to point out, that I never programmed as professional, and I am learning everything in school from distance learning for Bachelor degree, and I have no option to communicate with my teacher to discuss this problem. Thank you all very much for help in advance.

Edit:

namespace Semestrální_projekt___Půjčovna_vozidel
{
    [Serializable()]
    public class Vozidla
    {
        public string Vyrobce { get; set; }
        public string Model { get; set; }
        public int RokVyroby { get; set; }
        public int CenaVypujcky { get; set; }
        public int CenaProdejni { get; set; }
        public Vozidla(string vyrobce, string model, int rokVyroby, int cenaVypujcky, int cenaProdejni)
        {
            Vyrobce = vyrobce;
            Model = model;
            RokVyroby = rokVyroby;
            CenaVypujcky = cenaVypujcky;
            CenaProdejni = cenaProdejni;
        }
    }
        [Serializable()]
        public class Motorka: Vozidla
        {
            public int Kubatura { get; set; }
            public string Typ { get; set; }
            public Motorka(string vyrobce, string model, int rokVyroby, int cenaVypujcky, int cenaProdejni, int kubatura, string typ)
                :base (vyrobce, model, rokVyroby, cenaVypujcky, cenaProdejni)
            {
                Kubatura = kubatura;
                Typ = typ;
            }
        }
        [Serializable()]
        public class Automobil: Vozidla
        {
            public int ObsahMotoru { get; set; }
            public string Typ { get; set; }
            public string Prevodovka { get; set; }
            public int PocetMistSezeni { get; set; }
            public Automobil(string vyrobce, string model, int rokVyroby, int cenaVypujcky, int cenaProdejni, int obsahMotoru, string typ, string prevodovka,int pocetMistSezeni)
                :base(vyrobce, model, rokVyroby, cenaVypujcky, cenaProdejni)
            {
                ObsahMotoru = obsahMotoru;
                Typ = typ;
                Prevodovka = prevodovka;
                PocetMistSezeni = pocetMistSezeni;
            }
        }
    }