It is far safer and better to use a natural English plural when in doubt than to use a Latin or Greek plural. Only when the English plural is truly awful, such as basises or thesises, should you use the foreign plurals. But if in doubt, choosing the foreign plural to sound knowledgable can be a mistake. Here are some common errors:
Octopus – Greek root so the correct foreign plural would be octopodes, just use octopuses.
Ignoramus – idiomatic verb, not a noun. No foreign plural, use ignoramuses.
Processes – not of Greek origin. Do not pronounce the ‘es’ like ‘theses’ but like ‘recesses’.
Agenda – already a plural, use agendas.
