public class Course
{
    public int Id { get; set; }
    public string Title { get; set; }
    public double? Duration { get; set; }
    ...
    public Instructor Instructor { get; set; }
}

public class Instructor
{
    public int Id { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Email { get; set; }
    ...
}