There are many words in the English language that have been derived from foreign languages like Latin or Greek.
These words have two possible spellings for their plural forms: the English spelling and the foreign plural spelling.
Word: formula, Latin plural: formulae, English plural: formulas
Some words have only the Latin plural spelling.
larva – larvae; alga – algae
For words which have a Greek origin, we change them to their plural forms by replacing the -is at the end with -es.
analysis – analyses
crisis – crises