Verb conjugation of "seller" in French

In order to improve the language skills we changed some features on Vocabulix. From now on users can not only drill isolated lesson but adhere to a specific lesson plan. That means that they can set goals and seek help from other users.
In order to quickly become successful, one would have to change certain points, at least in my opinion: After 5 months of learning, I have still not found the grammar practice on which I should concentrate.
Many thanks for the corrections. Exactly what I needed to be happy. I want to learn French because it's a great language and because it sound so romantic to me. There is a lot more to improve.