在i上需要一个小于或等于<=，否则i永远不会大于或等于pigLatin. length（）。

for (int i = 0; i <= pigLatin.length(); i++) {
                    char firstChar = pigLatin.charAt(0); 
                    pigLatin = pigLatin.substring(1);
                    pigLatin = pigLatin + firstChar;
                    if (i >= pigLatin.length())
                    {
                        System.out.println(pigLatin);
                    }
                }

你的for循环条件是

i < pigLatin.length()

你说如果

if(i >= pigLatin.length()){....}

所以这个条件永远不会为真，因为没有输出，
参见下面的代码，

import java.util.Scanner;

/**
 *
 * @author rahmat waisi
 */
public class PigLatin {

    public static void main(String[] args) {
        try (Scanner scanner = new Scanner(System.in)) {
            while (true) {
                // System.out.print("Please enter a word: , Enter [ quit ] for exit : ");
                String pigLatin = scanner.nextLine();
                if (pigLatin.equals("quit")) {
                    break;
                }
                if (isVowel(pigLatin.charAt(0))) {
                    pigLatin += "ay";
                    System.out.println(pigLatin);
                } else {
                    String output = "";
                    int separation_index = findFirstVowel(pigLatin);
                    if (separation_index ==-1) {
                        System.out.println(pigLatin+"ay");
                        continue;
                    }
                    output+= pigLatin.substring(separation_index);
                    output+= pigLatin.substring(0, separation_index) + "ay";
                    System.out.println(output);
                }
            }
        }
    }

    private static boolean isVowel(char ch) {
        char v = Character.toLowerCase(ch);
        return v == 'a' || v == 'e' || v == 'i' || v == 'o' || v == 'u';
    }

    private static int findFirstVowel(String str) {
        for (int i = 0; i < str.length(); i++) {
            if (isVowel(str.charAt(i))) {
                return i;
            }
        }
        return -1;
    }

}

以下是一些输入：

pig
banana
trash
happy
duck
glove
eat
omelet
are
ffff
quit

它们的输出是：

igpay
ananabay
ashtray
appyhay
uckday
oveglay
eatay
omeletay
areay
ffffay

import java.util.Scanner;
public class two { public static void main（String[] args）{

System.out.println("Please enter a word");
    Scanner word = new Scanner(System.in);
    String pigLatin = word.nextLine();
    while (!pigLatin.equalsIgnoreCase("quit")) {
        if (isVowel(pigLatin.charAt(0))) {
            pigLatin = (pigLatin + "way");
            System.out.println(pigLatin);
        } 
        else {
            for (int i = 0; i < pigLatin.length(); i++) {
                char firstChar = pigLatin.charAt(0); 
                pigLatin = pigLatin.substring(1);
                pigLatin = pigLatin + firstChar;
                if (i >= pigLatin.length())
                {
                    pigLatin = pigLatin + "ay";
                    System.out.println(pigLatin);
                }
            }

        }

        System.out.println("Please enter a word");
        pigLatin = word.nextLine();
    }
    word.close();

}

private static boolean isVowel(char ch) {
    char v = Character.toLowerCase(ch);
    if (v == 'a' || v == 'e' || v == 'i' || v == 'o' || v == 'u') {
        return true;
    }

    else {
        return false;
    }

}

}