Violation of PRIMARY KEY constraint 'PK. Cannot insert duplicate key in object 'dbo.Cursuri'. The duplicate key value is (0)

Question

My assignment is to do crud operations using database first on an Windows Form app. Currently my save button (Salveaza) is not working properly ( I get the above error when I debug it) and I don't understand why.

namespace Database
    {
        public partial class EntityFrameworkCRUD : Form
        {
            Cursuri modelCursuri = new Cursuri();
            Studenti modelStudenti = new Studenti();

            public EntityFrameworkCRUD()
            {
                InitializeComponent();

            }

            private void btnCurata_Click(object sender, EventArgs e)
            {
                Clear();
            }

            void Clear() 
            {
                tbNumeCurs.Text = tbDescriere.Text = tbNume.Text = "";
                btnSalveaza.Text = "Salveaza.";
                btnSterge.Enabled = false;
                modelCursuri.CursID = 0;
            }

            private void EntityFrameworkCRUD_Load(object sender, EventArgs e)
            {
                Clear();
                this.ActiveControl = tbNumeCurs;

            }

            private void btnSalveaza_Click(object sender, EventArgs e)
            {
                modelCursuri.CursNume = tbNumeCurs.Text.Trim();
                modelCursuri.CursDescriere = tbDescriere.Text.Trim();
                modelStudenti.StudentNume = tbNume.Text.Trim();
                using (AplicariCursuriContext db = new AplicariCursuriContext())
                {
                    var cursnume = Console.ReadLine();
                    var cursdescriere = Console.ReadLine();
                    var studentNume = Console.ReadLine();
                    var cursurile = new Cursuri { CursNume = cursnume, CursDescriere = cursdescriere };
                    // var cursurile2 = new Cursuri { CursDescriere = cursdescriere };
                    var studentii1 = new Studenti { StudentNume = cursdescriere };

                    db.Cursuris.Add(modelCursuri);
                   // db.Studentis.Add(modelStudenti);
                    db.SaveChanges();

                }
                Clear();
                MessageBox.Show("Submited Successfully.");
     }
        }
    }

Also the designs of the tables I'm using with Sql Server for the tables Cursuri:

CREATE TABLE Cursuri(
    [CursID] INT NOT NULL IDENTITY,
    [CursNume] VARCHAR(50) NULL,
    [CursDescriere] VARCHAR(50) NULL,
    CONSTRAINT [PK_CursID] PRIMARY KEY ([CursID])
) 

And Studenti:

CREATE TABLE Studenti(
    [StudentID] INT NOT NULL IDENTITY,
    [StudentNume] VARCHAR(50) NULL,
    CONSTRAINT [PK_StudentID] PRIMARY KEY ([StudentID]),
    CONSTRAINT [FK_Studenti_Cursuri] FOREIGN KEY ([CursID]) REFERENCES Cursuri(CursID)
)