Theory:

A plural noun is a word that refers to that there is more than one person, animal, place, thing, or idea. To make a singular noun plural add 's', 'es', or 'ies' to the end.
Example:
Singular
Plural
abyss
abysses
alumnus
alumni
analysis
analyses
aquarium
aquaria
leaf
leaves
life
lives
loaf
loaves
man
men
mango
mangoes
memorandum
memoranda
mess
messes
axe
axes
Important!
There are a few exceptions to this rule, but not many.
Example:
1. A single moose is a moose, and a group of moose is still moose.
2. A single deer is a deer, and a group of deer are still deer
Important!
Click here to see more examples of irregular plural nouns.