fautes+de+français

About.com has a strong section on the French Language. Generally it is possible to read an explanation and then do an exercise to see if you have really understood the concept. Here are a few sections on items that are particularly confusing for English speakers:

[| "to leave" in French : Partir/s'en aller/sortir/quitter/laisser]

[|Depuis/pendant/pour]

[|C'est/il est]

[|Qui versus que]