ABSTRACT
Sometimes they obey their original plural form (crises); sometimes they assume an English plural (syllabuses, not syllabi) and sometimes either is possible (cactuses or cacti).
the following, which many people get wrong:
singular plural criterion criteria phenomenon phenomena curriculum curricula medium media
6 Invariable nouns Some nouns are used only in the singular:
music homework snow stuff
They should always be used with a singular verb even if the implicit sense is plural:
1 Music in every form delights me. 2 Snow has fallen, snow on snow.
