下面是您的代码的工作版本，只需进行最少的编辑:

#include <iostream>
using namespace std;

int grade(int mark){
  int gradepoint;
  if (mark >= 81) {
    gradepoint = 10;
  }

  if (mark >= 61 && mark <= 80) {
    gradepoint = 8;
  }

  if (mark >= 41 && mark<= 60) {
    gradepoint = 6;
  }

  if (mark <= 40){
     gradepoint = 0;
  }

  return gradepoint; 
}

int grade1(int mark1){
  int gradepoint1;
  if (mark1 >= 81) {
    gradepoint1 = 10;
  }

  if (mark1 >= 61 && mark1 <= 80) {
    gradepoint1 = 8;
  }

  if (mark1 >= 41 && mark1 <= 60) {
    gradepoint1 = 6;
  }

  if (mark1 <= 40){
     gradepoint1 = 0;
  }

  return gradepoint1; 
}

int grade2(int mark2){
  int gradepoint2;
  if (mark2 >= 81) {
    gradepoint2 = 10;
  }

  if (mark2 >= 61 && mark2 <= 80) {
    gradepoint2 = 8;
  }

  if (mark2 >= 41 && mark2 <= 60) {
    gradepoint2 = 6;
  }

  if (mark2 <= 40){
    gradepoint2 = 0;
  }

  return gradepoint2;
}


int main(){
  int mark, mark1, mark2;
  cout <<"Please enter your marks for English. ";
  cin >> mark;
  cout <<"Please enter your marks for Maths. ";
  cin >> mark1;
  cout <<"Please enter your marks for Science. ";
  cin >> mark2;
  cout << grade(mark1) + grade1(mark1) + grade2(mark2);
  return 0;
}

所以这是一次学习的经历，而不是复制/粘贴，下面是你做错的和/或可以做得更好的地方:

1. 作用域--gradepoint1，2和3在所有方法中都在局部作用域中。 您错误地认为编译器自动识别相似的变量名意味着相同的基础对象。 在此处阅读有关本地作用域的信息。
2. 您有同一个方法的多个副本--int grade()是一个配方。 您可以向它传递任何变量，并基于该配方检索输出。 这样做的目的是不要为传递给它的每个变量创建函数的完整副本。
3. 如果您想编写C++，这就不太符合C++-Y。 它是带有一些iostream的C。 实验类，重载等，以了解如何在C++中做得更好。 例如，您可以向名为GPaccalculator的类添加不同的功能，例如打印成绩，存储成绩，将GPA计算移到类内等等。

下面是经过大量编辑的代码的工作版本:

#include <iostream>
using namespace std;

int grade(int mark){
  int gradepoint;
  if (mark >= 81) {
    gradepoint = 10;
  }

  if (mark >= 61 && mark <= 80) {
    gradepoint = 8;
  }

  if (mark >= 41 && mark<= 60) {
    gradepoint = 6;
  }

  if (mark <= 40){
     gradepoint = 0;
  }

  return gradepoint; 
}


int main(){
  int mark, mark1, mark2;
  cout <<"Please enter your marks for English. ";
  cin >> mark;
  cout <<"Please enter your marks for Maths. ";
  cin >> mark1;
  cout <<"Please enter your marks for Science. ";
  cin >> mark2;
  cout << grade(mark) + grade(mark1) + grade(mark2);
  return 0;
}