Irregular plural nouns

Many English nouns have irregular plural forms. While the general rule says that adding -s or -es to a noun forms its plural, this is not always true. The following examples show some of the ways English nouns become plural.

For some nouns, like fish, there is no difference between the singular form and the plural form. Sometimes, however, people do use regular plural endings for irregular nouns, so in casual conversation you may hear fishes or elks. 

• One fish; three fish
  
• A deer; many deer
• An elk; some elk
• One species; several species
• A series; many series

Some English words come from Latin, and take a Latinate plural ending.

• A syllabus; all of the syllabi
• A larva; many larvae
• This genus; these genera
• Her thesis; their theses
• One crisis; two crises
• An index; a couple of indices
• A matrix; two matrices

Sometimes nouns end in -f or -fe and the f becomes a v before adding -s or -es.

• A loaf of bread; a few loaves of bread
• One wolf; a pack of wolves
• A knife; some knives

Some plurals have no pattern and have to be memorized.

• That person; those people
• One child; three children
• One mouse; two mice
• A goose; several geese
• A man; two men
• An ox; a pair of oxen

Some nouns do not have plural forms because they cannot be counted. These are called noncount nouns, or mass nouns. Some common noncount nouns are water, rice, sand, butter, mud, rain, advice, homework, progress, and music.

I hope this helps.