I'm using Java code as an example, but this can be answered if you don't know Java.

class Bird{
      public void sing(){
          System.out.println("Testing");
      }
}

class anClass extends Bird{
      public void sing(){    //If I remove the method than, `c.sing()` in main method (Polymorphism class) will print,"Testing"
          System.out.println("Testing second time");
      }
}

public class Polymorphism{
     public static void main(String[] args){
         Bird b = new Bird();
         b.sing();    //will print,"Testing"
         anClass c = new anClass();
         c.sing();   //will print,"Testing second time"
     }
}

class numberOne{
      public int add(int a, int b){
          return a+b;
      }
}

class numberTwo extends numberOne{
      public int sub(int a, int b){
          return a-b;
      }
}

public class Inheritance{
     public static void main(String[] args){
         numberTwo b = new numberTwo();
         b.add(5,3);    //will print,"8"
         c.sub(10,4);   //will print,"6"
     }
}

I had studied OOP, inheritance and polymorphism before, but I could never really get it.

In the examples above, I noticed that in the polymorphism examples, the methods don't return a value, while in the inheritance examples, they do. Would that be the difference between those two concepts?