Statements after Recursion Call in Java

I'm trying to learn recursion but am having a hard time understanding the following code.Specifically, When these two statements [remainLetters = insertAtIndex(remainLetters, tmpString, i);scramLetters = removeFromIndex(scramLetters, scramLetters.length() - 1) ]execute for the first time, remainLetters is set to be "d", making the remainLetters length of 1 while i = 0, but why didn't it enter the for loop afterwards since it satisfies the condition: i < remainLetters.length()? Thank you in advance!

public class WordScrambler {
/* Output every possible combination of a word.
   Each recursive call moves a letter from
   remainLetters" to scramLetters".
*/
public static void scrambleLetters(String remainLetters,  // Remaining letters
                                   String scramLetters) { // Scrambled letters
    String tmpString;      // Temp word combinations
    int i;                 // Loop index

    if (remainLetters.length() == 0) { // Base case: All letters used
        System.out.println(scramLetters);
    }
    else {                             // Recursive case: move a letter from
        // remaining to scrambled letters
        for (i = 0; i < remainLetters.length(); ++i) {
            // Move letter to scrambled letters

            tmpString = remainLetters.substring(i, i + 1);
            remainLetters = removeFromIndex(remainLetters, i);
            scramLetters = scramLetters + tmpString;

            scrambleLetters(remainLetters, scramLetters);

            // Put letter back in remaining letters

            remainLetters = insertAtIndex(remainLetters, tmpString, i);
            scramLetters = removeFromIndex(scramLetters, scramLetters.length() - 1);
        }
    }
}

// Returns a new String without the character at location remLoc
public static String removeFromIndex(String origStr, int remLoc) {
    String finalStr;      // Temp string to extract char

    finalStr = origStr.substring(0, remLoc);                     // Copy before location remLoc
    finalStr += origStr.substring(remLoc + 1, origStr.length()); // Copy after location remLoc

    return finalStr;
}

// Returns a new String with the character specified by insertStr
// inserted at location addLoc
public static String insertAtIndex(String origStr, String insertStr, int addLoc) {
    String finalStr;      // Temp string to extract char

    finalStr = origStr.substring(0, addLoc);                 // Copy before location addLoc
    finalStr += insertStr;                                   // Copy character to location addLoc
    finalStr += origStr.substring(addLoc, origStr.length()); // Copy after location addLoc

    return finalStr;
}

public static void main(String[] args) {
    
    scrambleLetters("word", "");
}

}