Why does car idles fine but stalls when put in gear?

If a car idles fine but stalls when put in gear, it may indicate issues with the fuel system, such as a clogged fuel filter or failing fuel pump, which prevent adequate fuel supply under load. Alternatively, it could be related to a problem with the ignition system, like worn spark plugs or ignition coils that can't handle the increased demand. Additionally, vacuum leaks or a malfunctioning idle air control valve could disrupt airflow and engine performance when engaging the transmission. Diagnosing the exact cause would require further investigation.