cerrar vs apagar
English 'turn off' has two Spanish forms. Cerrar closes taps, doors, accounts. Apagar switches off lights, devices, flames.
cerrar
To close or shut — doors, windows, taps, shops.
Cierra la puerta.
Close the door.
Stem change e→ie: cierro, cierras.
Cierra el grifo.
Turn the tap off.
Taps and gas are closed, not apagados.
Cerramos a las nueve.
We close at nine.
Shop opening hours.
apagar
To switch off electrical devices, lights, flames.
Apaga la luz, por favor.
Turn the light off, please.
The opposite of encender.
Apaga el móvil en el cine.
Switch your phone off in the cinema.
Devices. 'Apagar el ordenador' too.
Los bomberos apagaron el fuego.
The firefighters put out the fire.
Flames and cigarettes: apagar.
Tricky bit
'Cerrar la luz' is a common Latin American usage in some regions, but in Spain it sounds odd. And 'apagar el grifo' doesn't exist — taps are closed, not switched off.