Final answer:
The English rule for changing nouns ending in -ma to plural is usually to add -s, making it -mas. The -ess suffix is added to masculine nouns to form feminine nouns, sometimes with the deletion of a letter (e.g., waiter to waitress). Some nouns ending in -s appear plural but are singular for verb agreement, such as mathematics.
Step-by-step explanation:
The rule you are referring to typically deals with nouns ending in -ma that change to form the plural by replacing ma with -mas. However, you also emphasized the suffix -ess, which is used to create feminine forms of nouns, often when they end in the male counterparts -er or -or. An example of this would be the transformation of waiter into waitress, where it is notable that an e is often deleted if it comes before the -er in the male noun. Moreover, it is important to remember that in the English language, certain nouns ending in -s may appear to be plural but are in fact singular for the purpose of verb agreement; such as mathematics or news.