How to use if statements

Hello everyone,

I am a complete noob when it comes to Arduino programming, Basically I want , that if the user does not type anything in serial monitor for at least 3 seconds then the serial monitor prints "??" or something else.

I hope you all can help!

You should start with the examples which are aimed to n00bs. I am sure you can find answer there.
For any further advice it is better post your code.

How's this?

// https://forum.arduino.cc/index.php?topic=629508
// 2 august 2019
// remind user for serial input

int interval = 3000;
unsigned long startTheClock;
char receivedChar;

void setup()
{
  Serial.begin(9600);
  Serial.println(".... remind user for serial input ....");
  Serial.print("Compiler: ");
  Serial.print(__VERSION__);
  Serial.print(", Arduino IDE: ");
  Serial.println(ARDUINO);
  Serial.print("Created: ");
  Serial.print(__TIME__);
  Serial.print(", ");
  Serial.println(__DATE__);
  Serial.println(__FILE__);

  //turn off L13
  pinMode(LED_BUILTIN, OUTPUT);
  digitalWrite(LED_BUILTIN, LOW);

  Serial.println("Type something....");

} //setup

void loop()
{

  if (Serial.available() > 0)
  {
    receivedChar = Serial.read();
    Serial.print("You typed ");
    Serial.println(receivedChar);
    startTheClock = millis();
  }

  if (millis() - startTheClock >= interval)
  {
    Serial.println("Are you there?");
    startTheClock = millis();
  }
} //loop