What are singulars and plurals?

A singular is a word that tells us there is only one.
A plural is a word that tells us there are more than one.

For example:
cat is singular (there is only one cat)
cats is plural (there are more than one cat)

Different types of plurals

Most plurals are made by adding ‘s’ to the singular.
However, not all plurals are like this.

‘s’ plurals

SingularPlural
dogdogs
househouses
bottlebottles
bookbooks
computercomputers

‘es’ plurals

Nouns ending in ‘o’, ‘s’, ‘ch’, ‘th’, ‘sh’ or ‘x’ usually use ‘es’ for plurals:

SingularPlural
potatopotatoes
boss bosses
watchwatches
brushbrushes
boxboxes

However, this is not always true (for example, 1 path and 2 paths).

‘ves’ plurals

Nouns that end in one ‘f’ (or have an ‘f’ as the last sound) often change the ‘f’ to ‘v’, then add ‘es’:

SingularPlural
wifewives
wolfwolves
halfhalves
loafloaves
knifeknives

Again, this is not always true (for example: 1 reef, 2 reefs).

Unusual plurals

Some nouns in English have unusual plurals:

SingularPlural
womanwomen
personpeople
oxoxen
toothteeth
milleniamillenium

Unfortunately there is not an easy way to learn these; usually English speakers learn through experience.

No change plurals

There are some plurals (most often animals) that are the same as the singular:

SingularPlural
fishfish
sheepsheep
shrimpshrimp
speciesspecies
aircraftaircraft

Uncountable nouns are different

Singulars tell us there is one. Plurals tell us there are more than one.
But some nouns cannot be counted.
These uncountable nouns do not need plurals.

Uncountable nounDon’t sayDo say
water1 water
2 waters
some water
money1 money
2 moneys
some money
air1 air
2 airs
some air
meat1 meat
2 meats
some meat
happiness1 happiness
2 happiness
some happiness

Advanced note:
If talking about food, we usually only count if the food makes different meals on different plates.
I have some pizza. (pizza is general, and uncountable)
We bought 2 pizzas. (the pizzas are clearly separate)