Bad Plurals

English has a very useful but sometimes confusing (to non-native speakers) habit of using a noun as an adjective by placing it before another noun. For instance, a horse race is a race for horses. Note that in "a race for horses," I wrote "horses" in the plural, but in "a horse race," I used "horse" in the singular. This is because in this case, the qualifying noun becomes an adjective, therefore it is invariable, like all adjectives in English.

This gives us the following examples:

Long expression Shorter form with singular qualifier

The key for reading the symbols The symbol reading key

The security of buildings Building security

Files containing movies Movie files

Database of components Component databases

Long expression	Shorter form with singular qualifier
The key for reading the symbols	The symbol reading key
The security of buildings	Building security
Files containing movies	Movie files
Database of components	Component databases

The plural of words from Greek and Latin often keeps its original form. Thus:

one criterion, several criteria (not "one criteria," as one sometimes reads!)
one alumnus, several alumni (male ex-students)
one alumna, several alumnae (female ex-students)

The word software, much used by this author and his colleagues, is a collective noun which is invariable, unlike the French "logiciel." Therefore one cannot say "a software" or "we buy softwares from this vendor." When it is important to specify one or multiple software "units," you need to use another word or phrase: "program," "application," or "software package."