Here is a way to get a random word of a sentence ...
You might split the sentence (tb.Text) into a string [] of words by using string.Split(' '). Then get a random number between '0' and .Length of your words.
Then pick the random substring.
// split the string into substrings seperated by a blank ' ' ...
var strArrWords = tb.Text.Split(' ');
// get the number of words ...
var numWords = strArrWords.Length;
// get a random number between '0' and numWords-1
Random rnd = new Random();
var number = rnd.Next(0, numWords - 1);
// finally get your random word ...
var word = tb.Text.Split(' ')[number];
or the same in 2 lines ...
Random rnd = new Random();
var word = tb.Text.Split(' ')[rnd.Next(0, tb.Text.Split(' ').Length - 1)];
To visualize this, you can maybee open another textbox displaying your word, the user can edit it. Then you replace the word in the other box, depends ...
I hope that helps :-)