Quote Originally Posted by it-ogo
IMHO, to have one word in all cases is better idea.

Nom: Вот карандаш. Here is a pencil.
Gen: У меня нет карандаша. I have no pencil.
Dat: Добавь к карандашу. Add to a pencil.
Acc: Я вижу карандаш. I see a pencil.
Ins: Я пишу карандашом. I write with a pencil.
Prp: Я мечтаю о карандаше. I dream about a pencil.
Have you seen this lesson? It may be a good idea to combine them.
http://masterrussian.com/blcasetable.shtml