Collins Cobuild English Grammar (186 page)

R14
    In addition to the nouns mentioned above, there are words that are borrowed from other languages, especially Latin, and that still form their plurals according to the rules of those languages. Many of them are technical or formal, and some of those that are given below are also used with a regular
s
or
es
plural ending in non-technical or informal contexts. You may need to check these in a Cobuild dictionary.

R15
    Some nouns ending in
us
have plurals ending in
i
.

cactus		cacti
focus		foci
nucleus		nuclei
radius		radi
stimulus		stimuli

R16
    Some nouns ending in
um
have plurals ending in
a
.

aquarium		aquaria
memorandum		memoranda
referendum		referenda
spectrum		spectra
stratum		strata

R17
    Most nouns ending in
is
have plurals in which the
is
is replaced by
es
.

analysis		analyses
axis		axes
basis		bases
crisis		crises
diagnosis		diagnoses
hypothesis		hypotheses
neurosis		neuroses
parenthesis		parentheses