Is expiari an alternate form of the infinitive expiare?

Question

I take the following sentence from Fabules Faciles:

...hōc enim ūnō modō tantum scelus expiārī potuit

as "...only in this way could he atone for such a great crime." literally, "indeed in this one way for such an extent of crime to atone was he able". So, expiari seems to be an infinitive ("to atone"). However, I thought that the infinitive of expior was expiare, not expiari. Is it a subjunctive infinitive or something? This form, expiari, is not even listed in the grammar I use (https://www.online-latin-dictionary.com/).

Answer 1

You're correct that this is an infinitive! It's just the passive infinitive, rather than the active, so I would read scelus as the subject rather than the person atoning.

Rearranging the words into an English order rather than a Latin one, we get:

…enim hōc scelus potuit expiārī ūnō modō
…for this crime was able to be atoned for in only one way

The passive infinitives for the four conjugations are am-ārī, hab-ērī, dīc-ī, and aud-īrī. The third conjugation is the only weird one to learn; the others just replace the final -e of the active infinitive with -ī.