Do not know how to run forward the dc motors.

Using Arduino Motors, Mechanics, Power and CNC
6

the two motors moving forward with this code:

int value;
  for(value = 0 ; value <= 255; value+=5) 
  { 
    digitalWrite(M1,LOW);   
    digitalWrite(M2,LOW);       
    analogWrite(E1, value);   //PWM Speed Control
    analogWrite(E2, value);   //PWM Speed Control
    delay(30);

but when I change the code with

int value;
  for(value = 0 ; value <= 255; value+=5) 
  { 
    digitalWrite(M1,LOW);   
    digitalWrite(M2, HIGH);       
    analogWrite(E1, value);   //PWM Speed Control
    analogWrite(E2, value);   //PWM Speed Control
    delay(30);

in order to move one motor backward and the other motor forward
it can not work my setting M1 LOW and M2 HIGH, the motor stay still, what happen?