int prevInd = 0;
string lastWord = "";
private void richTextBox1_TextChanged(object sender, EventArgs e)
{
    if (richTextBox1.Text.Length == 0)
        return;
    if (richTextBox1.Text[richTextBox1.Text.Length - 1] == ' ')
    {
        lastWord = richTextBox1.Text.Substring(prevInd, richTextBox1.Text.Length - prevInd);
        prevInd += lastWord.Length;
        MessageBox.Show(lastWord);
    }
}

另一种方法，

Another way,

class Program
{
    static string GetLastWord(string dataToTest)
    {
        return String.IsNullOrEmpty(dataToTest) ? default(string) : dataToTest.Split(new char[] { ' ' }).LastOrDefault();
    }

    static void Main(string[] args)
    {

        string dataToTest = "Hello world I am a String!";
        string lastWord = GetLastWord(dataToTest);
    }
}

:)

:)