What is the difference between the imparfait and the passé composé?

To many students, this question is about as easy to answer as 'What's the sound of one hand clapping?'. This key difference between two important tenses is talked about in almost mystical terms. People who know the difference and can use the tenses correctly often can't explain it because it's instinctive, something they feel. And people's attempts to explain it have generated many myths.

The difficulty arises because:

1) the two tenses in French don't correspond neatly to equivalent tenses in English.

2) the difference is not bound by hard and fast rules: there are times when both might be     grammatically correct (although the meaning might be different, especially the     connotations, depending on which tense you choose).

3) the difference between them is conceptual: it relies on an understanding of aspect, which     is never taught in English classes, even though English is full of them

For an explanation of the differences, keep moving forward through the pages...