using System;

using System.Linq;

using System.Text;

​

public class Program

{

    public static void Main()

    {

        var strings = new[] { "firstName", "birthOfDate" };

        var sentences = strings

            .Select(GetSentence)

            .ToArray();

        foreach (var sentence in sentences)

        {

            Console.WriteLine(sentence);

        }

    }

​

    private static string GetSentence(string str)

    {

        var sb = new StringBuilder();

        sb.Append(Char.ToUpper(str[0]));

        foreach (var ch in str[1..])

        {

            if (Char.IsUpper(ch))

            {

                sb.Append(' ');

                sb.Append(Char.ToLower(ch));

            }

            else

            {

                sb.Append(ch);  

            }

        }

        return sb.ToString();

    }

}

​