Errors & Code Improving

// VERSION 1

package generalPractice1;
/*
 * Practice Problem:
 *   - There are a ton of students and you need to determine what letter grade each student got depending on their
 *     percentage.
 *     * Design your code as if there were a thousand students, not just the ones provided.
 *   - Print out how many of each letter grade there are using a count variable.
 *   - print each student's name along with their percentage and grade letter. One student per line.
 */

public class Main {
    public static void main(String[] args) {
        int[] grades = {30, 60, 85, 50, 90, 40, 90, 75, 99, 54};
        String[] students = {"Harry", "Josh", "Kyle", "Katelyn", "Tommy", "Jackson", "Jane", "Conner", "Paul", "Nancy"};
        checkGrades(grades, students);


    }

    public static void checkGrades(int[] grades, String[] students) { // this is like functions aka modules in Python.
        int countA = 0, countB = 0, countC = 0, countD = 0, countF = 0;
        String pluralCountA = "", pluralCountB = "", pluralCountC = "", pluralCountD = "", pluralCountF = "";
        String pluralCountA2 = "", pluralCountB2 = "", pluralCountC2 = "", pluralCountD2 = "", pluralCountF2 = "";
        String letterGrade = "";

        // Determines what each student's grade is for the part that is supposed to print student name, %, and letter grade.
        for (int i: grades) {
            if (grades[i] >= 90) {
                letterGrade = "A";
            }
            else if (grades[i] >= 80) {
                letterGrade = "B";
            }
            else if (grades[i] >= 70) {
                letterGrade = "C";
            }
            else if (grades[i] >= 60) {
                letterGrade = "D";
            }
            else if (grades[i] <= 59) {
                letterGrade = "F";
            }

            // Will go through the loop and each time run it for the corresponding student & their grade.
            System.out.println(students[i] + ", " + grades[i] + ", " + letterGrade);

            // Counts how many of each letter grade there are.
            if (i >= 90) {
                ++countA;
            }
            else if (i >= 80) {
                ++countB;
            }
            else if (i >= 70) {
                ++countC;
            }
            else if (i >= 60) {
                ++countD;
            }
            else if (i <= 59) {
                ++countF;
            }
        } // End of the for loop.

        // Determines what type of grammar to use in the println statement later on, depending on whether it is
        // singular or plural.
        if (countA < 2) {
            pluralCountA = "is ";
            pluralCountA2 = ".";
        }
        else {
            pluralCountA = "are ";
            pluralCountA2 = "s.";
        }

        if (countB < 2) {
            pluralCountB = "is ";
            pluralCountB2 = ".";
        }
        else {
            pluralCountB = "are ";
            pluralCountB2 = "s.";
        }

        if (countC < 2) {
            pluralCountC = "is ";
            pluralCountC2 = ".";
        }
        else {
            pluralCountC = "are ";
            pluralCountC2 = "s.";
        }
        if (countD < 2) {
            pluralCountD = "is ";
            pluralCountD2 = ".";
        }
        else {
            pluralCountD = "are ";
            pluralCountD2 = "s.";
        }
        if (countF < 2) {
            pluralCountF = "is ";
            pluralCountF2 = ".";
        }
        else {
            pluralCountF = "are ";
            pluralCountF2 = "s.";
        }
        System.out.println("There " + pluralCountA + countA + " A grade" + pluralCountA2);
        System.out.println("There " + pluralCountB + countB + " B grade" + pluralCountB2);
        System.out.println("There " + pluralCountC + countC + " C grade" + pluralCountC2);
        System.out.println("There " + pluralCountD + countD + " D grade" + pluralCountD2);
        System.out.println("There " + pluralCountF + countF + " F grade" + pluralCountF2);

    }
}