Activist is singular, activists is plural.
The plural of gymnasium is gymnasiums or gymnasia.
It's not really about the meaning; it's about singulating a plural word usually (I actually think always) ending in "ia":examples: bacteria (plural) bacterium (singular), media (plural) medium (singular).
"Fa'amafanafana le Atua ia te oe/outou." Oe is you singular; outou is also you, but plural.
It is myocardium.
Ta'utinoga - "He gave me an assurance that he won't do it again." "Na aumai lana (singular) ta'utinoga ia te a'u e na te le toe faia." "He gave me assurances ..." "Na aumai ana (plural) ta'utinoga ..."
a root word
Translating the idea, "E mafaufau lava ia na'o ia".
amnesia The words with the suffix -ia have the meaning of act/state
un veau (masc. - plural: des veaux) ia a calf in English.
Tetrahydridocarbon
BACTER-IA
a singer