These two tenses have very specific uses in French. Consider the uses of the Passé Composé:

The Passé Composé is used to express that a particular action is completed—however long it might have taken.
 

The Passé Composé is used to present a series of actions in the past (which you can only do if each action is completed):
 

The Passé Composé is used to indicate that an action occurred suddenly or that it interrupted another action:
 

Now consider the uses of the Imparfait:

The Imparfait is used to stress the ongoing nature of an activity in the past:

Whenever you can use the construction "was" plus "–ing" in English, the Imparfait is appropriate in French.

The Imparfait is used to express that a particular action happened repeatedly or habitually in the past:
 

The Imparfait is used to describe events, people, and objects:
 

The Imparfait is used to refer to physical, emotional, and spiritual states of being:
 

Usually the verbs être, avoir (when it means to "possess" or "to be"), pouvoir and vouloir are used in the Imparfait.

Sometimes you will see both the Passé Composée and the Imparfait used in the same sentence:
 

These are some guidelines that will help you determine which tense to choose when you are speaking or writing about the past. As in most cases, the context will give you clues as to what is more appropriate. The best way to master these tenses is to listen to many examples and to read as much as possible.

Examples of Passé Composé/Imparfait

• Non, mais Sylvie m'a déjà offert une armoire.
• Autrefois cet immeuble était une église.
• Martine prenait une douche quand le téléphone a sonné.
• Je voulais lui répondre, mais elle s'en est allée sans m'écouter.