[Fnoord]

You need to rehearse content or it will fade. However, if you know the answer 2 days later still, then it is in your long term memory (I saw a graph yesterday -probably from HN- where the recall percentage plummeted throughout the day but then stayed relatively stable). Anki has support for precisely this feature. When you start the card, you get to see dots in a color for a short while. This tells you how you perform on the card.

That being said, your question is specific to language (French in your example, but could just as well be code). Due to the complexity and possibilities you may not be able to recognize/cope with different combinations or situations.