ExtractSentences

namespace ExtractSentences
{
    using System;
    using System.Text.RegularExpressions;

    public class ExtractSentences
    {
        public static void Main(string[] args)
        {
            string text = "We are living in a yellow submarine. We don't have anything else. Inside the submarine is very tight. So we are drinking all the day. We will move out of it in 5 days.";
            string key = "in";

            for (int i = 0; i < text.Length; i++)
            {
                int currentDotIndex = text.IndexOf(".", i);
                string currentStr = text.Substring(i, currentDotIndex - i + 1);
                if (Regex.IsMatch(currentStr, @"\b" + key + @"\b."))
                {
                    Console.WriteLine(currentStr);
                }

                i = currentDotIndex + 1;
            }
        }
    }
}