1. Incomplete vs Complete
The imperfect describes an ongoing action with no specified completion:
J'allais en France. - I was going to France.
Je visitais des monuments et prenais des photos. - I was visiting monuments and taking pictures
The passé composé expresses one or more events or actions that began and ended in the past:
Je suis allé en France. - I went to France.
J'ai visité des monuments et pris des photos. - I visited some monuments and took some pictures.
2. Habitual vs Occasional
The imperfect is used for habitual or repeated actions, something that happened an uncounted number of times:
Je voyageais en France tous les ans. - I traveled (used to travel) to France every year.
Je visitais souvent le Louvre. - I often visited the Louvre.
The passé composé talks about a single event, or an event that happened a specific number of times:
J'ai voyagé en France l'année dernière. - I traveled in France last year.
J'ai visité le Louvre trois fois. - I've visited the Louvre three times.
3. Ongoing vs New
The imperfect describes a general physical or mental state of being:
J'avais peur des chiens. - I was afraid of dogs.
J'aimais les épinards. - I used to like spinach.
The passé composé indicates a change in physical or mental state at a precise moment or for an isolated cause:
J'ai eu peur quand le chien a aboyé. - I was scared when the dog barked.
Pour la première fois, j'ai aimé les épinards. - For the first time, I liked spinach.
4. Background + Interruption
The imperfect and passé composé sometimes work together - the imperfect provides a description/background info, to set the scene of how things were or what was happening (past tense of "be" + verb with -ing usually indicates this) when something (expressed with the passé composé) interrupted.
J'étais à la banque quand Chirac est arrivé. - I was at the bank when Chirac arrived.
Je vivais en Espagne quand je l'ai trouvé. - I was living in Spain when I found it.