Take random word from file to print sentence from BNF grammar

I am confused on what BNF grammar actually is. As of right now, I have a text file called lol.txt and my code prints out a random line in the file. However, I need to pick a adj, prep, noun, etc to make a random sentence. Is my logical approach wrong?

FileInputStream wordsFile = new FileInputStream("lol.txt");

BufferedReader br = new BufferedReader(new InputStreamReader(wordsFile));

    String[] strWordArray;
    strWordArray = new String[10];

    for (int i = 0; i < strWordArray.length; i++) 
    {
        strWordArray[i] = br.readLine();
    }
    wordsFile.close();
    Random rand = new Random();
    String Word = strWordArray[rand.nextInt(strWordArray.length)];
    System.out.println(Word);

}