Io lavoro perfino di domenica.

When do we use “perfino” and when do we use “neppure”? For now , I am only guided by the green letters input. Non è bene.

I have encountered this sentence too, and I recall it corresponding to “I work even on Sunday(s).”

Using “neppure” here to me would mean “I don’t even work on Sunday(s).” (as in “I generally don’t work, not even on Sundays”), but “pure” could be used to mean the same I think.

In fact, on Tatoeba the sentence “I work even on Sunday.” has translations containing both “perfino” and “pure”, and also a third and fourth option with “anche” and “persino”.

My personal preference would be to have these options automatically appear under “Alternative answers” (at least as a toggle-able option, so it isn’t forced upon everyone), and in fact there are some collections where all linked sentences with the same meaning, but different words, appear after solving the cloze (and can also result in the text colouring green for these options too).

You are able to add “Alternative answers” yourself though, in which case, the next time you play it, you will find you can add whichever one you prefer.