In French, to express an action or event in the past you may need to use two tenses together: l'imparfait, and le passé composé.
I have studied the past tenses but I'm not sure when to use them, could you help me?
Mais oui, bien sûr ! Let's have a look:
The past tenses in French are like going to the theatre!
What is happening on the stage in front of you is like the passé composé (you can see and follow the action).
But what is happening on the background is like the imparfait (it is important information - costumes or decor - relevant to understand the story but not mandatory for the action to continue).
Let's look at an example:
Quand je suis entré mes enfants faisaient des crêpes dans la cuisine. Il y avait de la farine partout. Nous avons mangé les crêpes ensemble et ils ont nettoyé la cuisine pendant que je lisais sur le canapé.
When I entered, my children were making crêpes in the kitchen. There was flour everywhere. We ate the crêpes together and they cleaned while I was reading on the sofa.
Do you see the difference between both tenses now?
Mmmh... It's like the theatre, right?
A little, yes. Think about the action they do and picture yourself doing them:
Quand je suis entré mes enfants faisaient des crêpes dans la cuisine.
When I entered, my children were making crêpes in the kitchen.
They are used together to express two simultaneous actions?
Yes, and... ?
Il y avait de la farine partout.
There was flour everywhere.
Imparfait describes the setting?
Oui: the decor, the landscape, the situation, the people in the background, their state of mind, etc. Bravo ! Et enfin... ?
Nous avons mangé les crêpes ensemble et ils ont nettoyé la cuisine pendant que je lisais sur le canapé.
We ate the crêpes together and they cleaned while I was reading on the sofa.
Looks like the passé composé in here is used to talk about the main actions, the ones happening on stage, and the imparfait the side actions, or the ones happening in the background.
Absolument ! We say the main actions or the events, what makes the story continue. So, you get it now?