Conjugation nation is a decent app, but it lacks a vital mood for conjugation; the negative imperative. Although it is classed under imperative, it conjugates differently from giving positive commands and needs to be practiced in its own right.
Especially in the most important commands (to the informal second person singular and plural)

Habla!
No Hables!!

Hablad!
No Habléis!

And it's also good for users to recognize and learn that the formal moods do not change between negative and positive, a mistake learners can make.

It's important to know this by heart in Castellano, so it would be very handy for a lot of us here in Spain if you could add it.