-a in Latin

The ending -a in Latin is ambiguous.

In some contexts, it shows a feminine singular. So an alumna is one female former student, where an alumnus is a male. Their plurals are alumnae and alumni respectively. (In Latin, if there are both males and females, one should use the masculine form. So a group of former students from a mixed-sex school is its alumni. They may refer to their alma mater, 'benign mother', which shows another feminine singular.)

In other contexts, the ending -a shows a neuter plural. Words that ended in -um in Latin formed their plurals in -a. These include such words as curriculum, which gives the plural curricula; and an adjective curricular (don't allow your spellchecker to give you the wrong choice here!). Some of the words in Latin ending in -us form plurals in -era, but the commonest plural form for nouns like this is -i.

A particular kind of neuter plural comes with the endings '-anda' and '-enda', which means roughly 'the things [i.e. the neuter plural] which should be done'. Corrigenda and emendenda are sometimes given at the front of a reprinting of a book - they mean the things that should be corrected and emended. In Latin, the singular would be corrigendum and emendendum - but these words are rare in contemporary English, even among the most academic of academics. Many ordinary people talk of referendums; academics prefer referenda.

There is also an -a ending associated with the past, or passive, participle. You may recognise this from the words that have been adopted from Latin into English, where it often appears in the form of an -ate ending for adjectives or nouns. Such words as 'graduate' for instance represent a Latin graduat*, 'one who has taken a degree'. With an -us ending in Latin, this indicated a male person; with the ending -a, it was a female.

But it could also show a plural number of things.