What's the problem? My motor works properly only when we press the left key and not others int mf11=13;//motor1 1st terminal  
int mf12=12;//motor1 2nd terminal
int mf21=9;//motor2 1st terminal 
int mf22=8;//motor2 2nd  terminal 
int fr=2;//front key 
int bk=3;//back key 
int rt=4;//right key  
int lt=5;//left key

void setup() {   
    pinMode(mf11,OUTPUT);
    pinMode(mf12,OUTPUT);
    pinMode(mf21,OUTPUT);
    pinMode(mf22,OUTPUT);
    pinMode(fr,INPUT);
    pinMode(bk,INPUT);
    pinMode(rt,INPUT);
    pinMode(lt,INPUT);
}

void loop() {  
    int s = digitalRead (fr);
    int h = digitalRead (bk);
    int i = digitalRead (rt);
    int v = digitalRead (lt);
    if(s==HIGH) {   
         digitalWrite(mf11,HIGH);
         digitalWrite(mf12,LOW);
         digitalWrite(mf21,HIGH);
         digitalWrite(mf22,LOW);
    }   
    if (h==HIGH) {   
        digitalWrite(mf11,LOW);
        digitalWrite(mf12,HIGH);
        digitalWrite(mf21,LOW);
        digitalWrite(mf22,HIGH);
    }
    if (i==HIGH) {   
         digitalWrite(mf11,LOW);
         digitalWrite(mf12,HIGH);
         digitalWrite(mf21,HIGH);
         digitalWrite(mf22,LOW);
    }
   if (v==HIGH) {  
        digitalWrite(mf11,HIGH);  
        digitalWrite(mf12,LOW);  
        digitalWrite(mf21,LOW);  
        digitalWrite(mf22,HIGH);
   } else {   
       digitalWrite(mf11,LOW);
       digitalWrite(mf12,LOW);
       digitalWrite(mf21,LOW);
       digitalWrite(mf22,LOW);
  }
}
The reason your code only works for left, is because when v is LOW your else block gets executed and sets all pins to low! You need to use else if.

if(condition1)
{
  ...
}
else if (condition2)
{
  ...
}
else
{
  ...
}