Try this (and see my comments):
int sensor = 0;
int ledPin = 5;
int val2;
void setup(){
pinMode(ledPin, OUTPUT);
Serial.begin(9600);
}
void loop(){
int val = analogRead(sensor);
Serial.println(val);
delay(100);
if (val<1020) {
digitalWrite(ledPin, HIGH);
}
else {
digitalWrite(ledPin, LOW);
}
if (val>1020){
val2=0;
}
else if (1000<val){ // will only reach here if val <= 1020 so no need to test
val2=1;
}
else if (910<=val){ // will only reach here if val <= 1000 so no need to test
val2=2;
}
else if (898<val){ // will only reach here if val < 910 so no need to test
val2=3;
}
else if (810<=val){ // will only reach here if val <= 898 so no need to test
val2=4;
}
else if (800<val){ // will only reach here if val < 810 so no need to test
val2=5;
}
else if(740<val){ // will only reach here if val <= 800 so no need to test
val2=6;
}
else { // will only reach here if val <= 740
val2=7;
}
} // Don't forget the closing brace for loop()...
Also, if ... else isn't a loop, it's a conditional control structure. Please watch your terminology to keep confusion down.