I, too, would like to know why it is 'īnfīnītīvus passīvī' rather than ''īnfīnītīvus passīvus' --- and I have a related question:
On the previous page (p. 74, Cap. X, lines 133, 134, and 154) he has 'Īnfīnītīvus' divided into 'Āctīvum' and 'Passīvum', where I would expect to see 'Āctīvus' and 'Passīvus'.
I'm guessing that 'īnfīnītīvus' is masculine to agree with the implied noun 'modus' (mood, voice) which it modifies. But in both cases, why do the forms 'Āctīvum' and 'Passīvum', and (from the original post) 'passīvī', seem not to agree with '(modus) 'īnfīnītīvus'?
Ho! I think I've answered my question about 'Āctīvum' and 'Passīvum' by peeking at Cap. XXXV, Ars Grammatica: they're neuter because they modify the (implied) neuter noun 'genus' (which here, I think, must be understood as 'voice' rather than 'gender').
From Cap. XXXV, lines 60 ff.:
Modī verbōrum sunt indicātīvus, imperātīvus optātīvus, coniūnctīvus, īnfīnītīvus.
Lines 65 ff:
Genera verbōrum sunt āctīva, passīva, neutra, dēpōnentia (though here I would expect 'āctīvum, passīvum, neutrum, dēpōnentium', so I'm still a bit puzzled).
Aha! It's because they modify (implied) 'vōx' (voice), which is feminine! So, then, 'genus' (above) did not mean 'voice' or 'gender', but 'kind'.
But then back on p. 74, 'Āctīvum' and 'Passīvum' is still a puzzle.
And now I'm more than ever confused about the original question, 'īnfīnītīvus passīvī'.
I think that in both cases, the solution must rest on the fact that there are implied nouns which are being modified by 'Āctīvum', 'Passīvum', 'passīvī', and which they have to agree with, but I can't figure out what those nouns are!
And a look at
THREAD: latin-grammatical-terms tells me that 'genus' does indeed mean 'voice'.
Feeling completely confused now.