A new article added to the grammar section of the French Linguistics web site looks at the question of when to use an and when to use année to translate the word "year" in French. To the English speaker (and indeed in some cases to native French speakers), the issue of which of these two synonyms to use can be surprisingly complex. The articles attempts to summarise this complexity by highlighting various conditions that tend to favour either an or année. For example, the use of an adjective or quantifier strongly favours année, whereas the use of a "normal" cardinal number strongly favours an. (For what happens when you have a number and an adjective, see the article!)
The article touches on an additional difficulty that advanced students may encounter with these words: their use has actually changed over time, including relatively recently. Therefore, readers of some of the French "classics" are liable to come across uses of the word an in particular which are now obsolete.