It would be very very helpful to add the feature of having the option to work with stem changing verbs or not.

When I first began to study Spanish, I found this app and thought i was going to love it. However, I could not use it for a very long time because I was a beginner to verbs and was unfamiliar with many verbs that were stem-changers and was not ready to work with them. Identifying them is still sometimes a problem and I have had much much practice!

Someone might say "well you can can go to the list of verbs and individually choose stem changing verbs and test on them"
This is true. But... When you are learning new verbs and you see
Volar
Pedir
Mentir

One would not know by the look of the verb in it's infinitive form that these are

Volar O>UE
Pedir E>I
Mentir E>IE

There is the option to
"Select by stem"
-AR Verbs
-ER Verbs
-IR Verbs
Deselect all

A similar option would be great for stem changers in the many tenses(:

Overall, 5 star app for sure! This app is very handy.