In English, we use the same word whether we mean a specific point in time ("this morning") or the duration of that time ("the whole morning"). In French, four common time periods have two distinct forms: a short masculine form for a point in time, and a longer feminine form for a duration of time.
Points in time and units: an, jour, matin, soir
Use the shorter masculine words (an, jour, matin, soir) when you are treating time as a simple unit of measurement, marking a specific point in time, or describing a routine.
You also use the short form an when stating someone's age or counting a strict number of years:
Duration and experience: année, journée, matinée, soirée
When you want to emphasize the length of time, the experience, or what actually happened during that period, use the longer feminine forms ending in -ée (année, journée, matinée, soirée).
These are the forms you must use with expressions like toute la (all the) or pendant la (during the):
You also use these feminine forms when describing the quality of the time with an adjective:
Finally, keep in mind that while you use an for counting specific years, année is the standard choice for most general statements about years, such as "every year" (chaque année):
