I need more work on irregular verbs. Maybe you could add a 'select irregulars' in verb settings, with options for 'ir' 'er' 'ar' endings in spanish.

I have big fingers and would very much appreciate the larger keyboard

It would be great to allow a rewind while doing the test, especially to access the conj.-table in case of an incorrect answer. While the correct form is displayed after an incorrect answer, I find it even more helpful to have the full reference for the selected tense.

It's confusion to have "Je" when the following verb is starting by a vowel.
Could you put an option : "Je/j'"
A larger list of verbs.

I love that you get prompted with the pronouns, and that you can choose all or one of them.  But, what do you think about adding to that list with more "real world" things.  For example, "Eduardo y yo: <infinitive>  Obviously, you'd want to choose the Nosotros form, but it makes you think a bit harder and its a bit more inclusive of they types of statements you'd use with the language.  Similarly, "Eduardo y Rita: <infinitive>" for Ellos/Ellas/Uds.  There are probably a few other real-world examples for each form, but I think you get the picture.

Multiple choice answers for quicker sessions

The only thing I would suggest is to make a Brasilian Portuguese version - & maybe have a button for English translation - like verb2verbe

It would be nice if there were a view that listed which verbs are currently selected for testing. Consider that the only way to see which verbs are selected is to scroll through a very long list of verbs.

Spanish second person present subjunctive ends in "s". In the dictionary you have it correct but in the quiz it is often marked as incorrect. The answer shown as correct (but is actually incorrect) is second person familiar imperative form.

I learned that the third person singular of "vendere" in the passato remoto is "vende" with a grave accent on the last "e" and not the acute accent as Conjugation Nation shows.

I checked my Barron's 501 Italian Verbs and it shows the grave accent. However, I checked some sites online and I have found both. :(

I have a screenshot of the error if you want to see it.

Maybe even note things like irregular in the future, irregular in all tenses, regular but spelling changes (like manger in French)