Nice job. Think about adding a place where you can select the words you want to practice. Or a word cloud with the words available to practice.

Also, the hardest part about using a site like this is using it on a regular basis. Think about gamefying the site so the user feels a need to continue.

It's hard to suggest improvements unless you use the site on a regular basis so you make sure you ask your users how to improve it since they are the ones that would know best.