In English, the digraph ae functions chiefly as a suffix to denote the plural of Latin borrowings that retain a hint of their original “Latin-ness,” chiefly in the realm of scientific vocabulary.
The rule for forming the plural of scientific terms ending in –a is to change the –a to –ae. Ordinarily, the pronunciation of this –ae is long e [ee].
Note, the hard g [/g/] in alga [al-gah] becomes a soft g [/j/) in algae [al-jee].
I once noticed a television ad for a flea-killing product for pets. The principal message was that the product “doesn’t just kill adult fleas, but eggs and larvae as well.” The first time I saw the ad, the voice-over pronounced what was clearly meant as the plural larvae as [lar-vuh]. On a subsequent airing of the same ad, the pronunciation had been changed from [lar-vuh], but not to [lar-vee]. This time, it was pronounced [lar-vay].
Charles Elster, known for his often curmudgeonly The Big Book of BEASTLY Mispronunciations, admonishes the careful speaker to avoid pronouncing the –ae as either long i [lar-vye] or long a [lar-vay]
Merriam-Webster shows the first pronunciation for pupae with a long e [piu-pee], but gives a second pronunciation with long i [piu-pye].
Elster devotes an especially long commentary to the plural of vertebrae. He definitely does not want to hear it pronounced as [ver-tuh-bray]. I suspect that he would find the pronunciation guide to this word in Merriam-Webster especially disheartening. Three pronunciations of vertebrae are given: 1. [ver-tuh-bray], 2. ver-tuh-bree], 3. [ver-tuh-brah]. The plural vertebras is also listed.
Several of these Latinate words have alternative –s plurals in addition to their –ae endings, depending upon context. For example, formulae is used in some scientific writing, but formulas has become the norm in most other contexts.
Here are some words that are used with either the –ae or –s plural.
Another –ae plural that Charles Elster feels strongly about is alumnae, a woman who has graduated from a particular school, college, or university. The singular is alumna. Again, M-W would be a disappointment to him. The first pronunciation listed is [alum-nee], but a second choice is also given: [alum-nye].
The masculine form of alumna is alumnus. The plural of alumnus is alumni. The masculine plural alumni can include women graduates. For that reason, co-ed universities refer to their graduates as alumni [alum-nye]. An all-women’s college, on the other hand, will refer to its graduates as alumnae [alum-nee]. In this age of gender consciousness, the traditional words alumnus, alumna, alumni, and alumnae may all combine in the gender-neutral shortening, alum, with alums as the plural.
If the pronunciation of the suffix –ae can be contentious, the pronunciation of ae in other parts of a word can be downright perplexing.
M-W gives both [sun-day] and [sun-dee]
Both OED and M-W give [dee-mon] as the only pronunciation. However, owing to the use of the word to refer to good spirits in some kinds of fiction, many readers pronounce it [day-mon] to distinguish it from demon, a word for bad spirits.
The OED makes no difference between the pronunciation of fairy and faerie. M-W, however, differentiates between fairy [fer-ee] and faerie [fay-ree].
This word for a high nest can be pronounced [air-ee] or [eer-ee].
Elster says, “Do not say [AY-jis],” but M-W gives [ay-jis] as the first pronunciation and Elster’s preferred [EE-jis] second.
Finally, we have Caesar [see-zer], Maeve [mayv], and maestro [my-stro]
Bottom line: When in doubt, look it up.