#latin words in English:

ending in -ex or -ix in the singular, end in -ices in the plural
▫️index - indices; appendix - appendices.

-um in the singular, -a in the plural
▫️datum - data; medium - media; stratum - strata.

-us / -ii
▫️radius - radii

-a / -ae
▫️agenda - agendae; formula - formulae.