What are the rules in changing foreign word to plural nouns?

There isn't a single, universally applicable rule for changing foreign words to plural nouns. The plural forms of foreign words often depend on their original language and how they've been assimilated into English. Here's a breakdown:

1. Latin and Greek Words

* Common endings:

* -us -> -i: alumnus -> alumni, stimulus -> stimuli

* -a -> -ae: alumna -> alumnae, formula -> formulae

* -um -> -a: datum -> data, medium -> media (but "mediums" is also accepted for spiritualists)

* -on -> -a: phenomenon -> phenomena, criterion -> criteria

* -is -> -es: analysis -> analyses, axis -> axes

* Exceptions:

* Some words ending in -us can be made plural with -es: octopus -> octopuses, virus -> viruses

* Some words ending in -um can be made plural with -ums: stadium -> stadiums, curriculum -> curriculums

* Some words have irregular plurals: octopus -> octopuses or octopi, focus -> foci

2. French Words

* Common endings:

* -eau -> -eaux: bureau -> bureaux

* -eu -> -eus: tableau -> tableaux

* -ou -> -ous: couteau -> couteaux

* Exceptions:

* Some words remain singular in English: attorney general, chateau

* Some words are pluralized with "-s": frere -> freres

3. Italian Words

* Common endings:

* -o -> -i: soprano -> soprani

* -a -> -e: camera -> camere

* Exceptions:

* Some words have irregular plurals: basso -> bassi, bambino -> bambini

4. Spanish Words

* Common endings:

* -o -> -os: taco -> tacos

* -a -> -as: fiesta -> fiestas

* Exceptions:

* Some words have irregular plurals: el niño -> los niños, la niña -> las niñas

5. German Words

* Common endings:

* -chen -> -chen: kindergarten -> kindergartens (Note: "kindergartens" is more common than "kindergarten")

* -lein -> -lein: fräulein -> fräuleins

* Exceptions:

* Many words ending in -er, -ing, -ung, -heit, -keit, -schaft are pluralized with "-s": der Führer -> die Führer, das Ding -> die Dinge, das Lager -> die Lager

6. Other Languages

* -s: The most common plural form for many foreign words, especially those that have become thoroughly anglicized.

* -es: Used for words ending in -s, -x, -z, -ch, or -sh, following general English rules.

Tips

* Consult a dictionary: When in doubt, always consult a dictionary for the correct plural form.

* Be mindful of context: Some words may have different plural forms depending on their specific meaning or usage.

* Consider the level of formality: Some plural forms are more formal than others.

* Be consistent: Use the same plural form throughout your writing to avoid confusion.

Remember, these are general guidelines, and there are always exceptions. It's best to double-check the plural form of any foreign word you're unsure about.