Different Types of Past Tense
The past tense — it is used to talk about something that has already happened. Some examples are;
I went to a movie yesterday.
I’ve read that book before.
I used to ride my bike to school every day.
These are all events that have happened in the past, and are no longer happening. In French, as in English, there are multiple ways to talk about the past.
Two of the most common tenses used to do so are the passé composé (literally, "compound past") and the imparfait (literally, "imperfect").
While you may seen them both used in the same sentence, and any verb can be conjugated in one or the other, they have very different uses.
The Passé Composé
This is usually the first past tense taught in French grammar books. You know how to form it, but what specifically does it mean? There is not an exact equivalent in English, but our closest tense is the simple past. I ate. He read. She won. You understood. The passé composé represents a one time event in the past, something that has both begun and ended already in our story.
Like the passé composé, there is not an exact equivalent of this tense in English. The majority of the time, it is translated with either the "was/were + [verb]-ing" or the "used to [verb]" tenses, though it can be others as well. I was driving my car. We used to eat ice cream on Sundays. It was a cloudy day. These are all sentences that would use the imparfait in French. This tense represents an ongoing event in the past, something that is currently taking place for our story.
Comparing the Two
Here is a short story in French: Quand j'étais jeune, le samedi ma famille et moi allions au ciné. Une fois, on a vu "Les Misérables" avec Gérard Depardieu. This story has three different verbs in it, two in the imparfait and one in the passé composé. The translation reads as follows: When I was young, on Saturdays my family and I used to go to the movies. One time, we saw "Les Misérables" with Gérard Depardieu.
The two verbs in the imparfait, étais and allions, are background information. They are descriptions of a scene, and as such, are not discrete events that take place at an exact moment, but rather over a long period of time, and that is why they are in the imparfait. Background description, scenery, and such; these are almost always in the imparfait. The verb in the passé composé, a vu, is a one time event.
We went to the movies every Saturday, thus imparfait, but just the one time, we saw Les Misérables, so it is in passé composé.
If you visualize a timeline, events in passé composé would be like single dots on the timeline. They are single events, and take place once, at a known time. Events in imparfait are are long lines along our timeline. They last for a period of time, and usually take place around our story, giving a description.
Another way to look at the distinction is to remember that any verb can be conjugated in both tenses. In English, take the sentences "We drove to the movies," and "We were driving to the movies."
While the sentences are identical, other than the verb tense, the first one is understood to be a single event, and you don’t expect to hear more about what happened during the trip, but rather what happened after the trip. With the second sentence, however, you are expecting to hear something more about what happened during the trip. This is very similar to the difference between passé composé and imparfait.
There are some generalities that can help determine which tense to use. Background, description, surrounding or repeated events; they are usually imparfait. One time events, events that are completed, or ones marked with words such as "once" or "suddenly,"; they are usually passé composé. But truly understanding the difference between them comes naturally, like tenses in English, and these examples should help make the difference clearer.