Understanding When to Use "Pendant un An" or "Pour un An"

Mistakes will always be made in French, and now you can learn from them.

Pour: Duration in the Future Only

The French preposition pour usually means "for," and it has a few other possible meanings as well, such as the duration of an event in the future:

   Je vais y habiter pour un an. > I'm going to live there for a year.

   Il parlera pour une heure. > He will speak for an hour.

   Je serai en France pour un an. > I'll be in France for a year.

Pendant: Duration in the Past, Future, Always

But you can't use pour to express the entire duration of a past event over a period of time. That is reserved for pendant, one of several French temporal prepositions, which also include à, en, dans, depuis and durant. 

You can use pendant as well when talking about something in the future, if you want to stress the length of time: 

   Je vais travailler seulement pendant quatre heures aujourd'hui.
   I'm only going to work for four hours today. 

In other words, pendant can always be used, but pour can only be used for the future.

   J'étais en France pendant un an.
   I was in France for a year.

   J'ai étudié le français pendant un semestre.
   I studied French for one semester.

   Nous avons parlé pendant des heures.
   We talked for hours.

Additional Resources

Temporal prepositions
All about pour