Learn French: J'allumais la lumière quand j'ai entendu un cri. - I was turning on the light when I heard a scream.

This sentence shows a classic interrupted action. J'allumais (I was turning on) is the ongoing background action, so it uses the imparfait. J'ai entendu (I heard) is the sudden, specific event that happened in the middle of it, so it uses the passé composé.

Does the word quand usually trigger the passé composé?

In sentences like this, yes! Quand means 'when' and is often used to introduce the sudden, specific event that interrupts your background action. You will see the pattern 'imparfait + quand + passé composé' very often when telling stories in French.

I sometimes mix up entendre and attendre. How can I remember which one is used here?

Entendre means 'to hear,' which is why j'ai entendu means 'I heard.' It is easy to confuse with attendre, which means 'to wait.' A helpful trick is to remember that entendre has an 'e' for 'ear'.

Can I use allumer for turning on anything, like a TV or a car?

You can use allumer for things that produce light, heat, or run on electricity, like a TV, a lamp, or a fire. However, to turn on a car engine or start a machine, you would normally use démarrer (to start).